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by hughes on February 15, 2006, 02:56:00 PM


Intro

Top-end rebuilding is the most frequent and costly service routine on two-stroke dirt bikes. Every year, dirt bike riders waste loads of money on top-end parts that didn't need to be replaced, or make costly mistakes while performing repairs. This section will give you the dos and don'ts to easy top-end rebuilding, plus some tips that aren't printed in your factory service manual.

Before You Start

Thoroughly wash your bike because dirt stuck to the underside of the top frame tube could break loose when servicing and fall into the engine! Use a stiff plastic brush and hot soapy water to clean off the grit and grime around the base of the cylinder, on the carburetor and intake boot, and especially underneath the top frame rail. Degreaser can be used on metal surfaces, but take care not to leave it on rubber or gasket surfaces.

Tools

You'll need at least some 3/8-inch-drive metric sockets and box wrenches (open-end wrenches will round off the edges on the cylinder or head nuts, and shouldn't be used for top-end rebuilding), a needle-nose pliers for removing circlips, and a gasket tool to scrape the old gaskets away. For soft tools, get some shop towels, aerosol oven cleaner, a Scotch-Brite pad, a locking agent such as Loctite, a gasket scraper, a brush, and a bucket of soapy water. Regarding measuring tools, you'll need a compression tester, a feeler gauge, and a digital vernier caliper.

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Compression Testing

A compression tester is a useful diagnostic tool, and readily available from Sears or auto parts stores. Buy the threaded type and make sure the kit comes with an adapter that matches the spark plug threads of your engine. Performing a compression test is simple. Start by removing the spark plug, thread in the adapter, and hold the throttle wide open and the kill button on. This will prevent any spark and enable the engine to draw in maximum airflow. Then kick-start the engine several times until the needle on the pressure gauge peaks. The pressure reading depends on two main factors; the compression ratio and the altitude at which the engine is being tested. The compression ratio will also depend on if the engine is equipped with exhaust valves and their condition. When the exhaust valves are in the closed position the compression ratio will be greater than if the valves are carbon-seized in the open position. The difference may yield a pressure reading 25 psi. The quality of compression testers varies greatly. The main thing that a compression tester can identify is a change in condition. Whenever you rebuild the top end, take a compression pressure reading and mark it down. When the pressure changes 20% check the condition of the piston and rings. Pistons usually last twice as long as rings.

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Crankcase Pressure Testing

The crankcase of a two-stroke engine is sealed from the tranny. It's important that the two crankshaft seals be in optimum condition. One side of the crankshaft uses a dry seal and the other a wet seal. The dry seal runs on the magneto side and the wet seal runs in oil on the tranny side. When the dry seal wears, the crankcase sucks in hot air, causing the mixture to run lean and overheat the engine. When the wet seal wears, the crankcase sucks in tranny oil, causing the engine run rich and eventually wet-foul the spark plug.

A crankcase pressure test involves the use of a vacuum pump with spark plug adapter, and rubber plugs to block off the intake and exhaust manifolds of the cylinder. The piston must be positioned at BDC to allow the transfer ports to be wide open linking the bore and the crankcase. The hand-pump produces vacuum pressure up to a standard setting of 5 psi. The normal bleed-down pressure loss is 1 psi per minute. Cylinders with complicated exhaust valve systems can be difficult to block-off air leaks, and harder to test. Crankcase pressure testing kits are available from Motion Pro.

If I suspect that an engine has an air leak in the crankcases, I do a visual test. Start by power washing the engine clean. Then remove the magneto cover. Spray the magneto clean with an aerosol can of brake cleaner. Make sure to use a non-chlorinated type of cleaner (green colored can). Now spray baby powder to all the suspect areas of the engine. Spray the powder on the crankcase around the magneto, at the crankcase seam line, the cylinder base, and the reed valve. Run the engine for a while, the white baby powder will highlight any fluid or air leaks on the engine. The baby powder test is much better than the alternative test of blowing raw propane gas at different areas of a running engine and listening for a change in the idle rpm. That is dangerous because it involves flammable gas and a hot engine with random electrical shorts.

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Maintenance and Inspection

A thorough top-end rebuild requires removing the reed valve, cylinder head, and cylinder. You should tear down your top end periodically and inspect the reed valve, cylinder head, cylinder, piston, and so on. Use the following chart to determine when you should tear down your bike:

Displacement: 80cc 125cc 250cc 500cc

Tear down after: 5 hours 10 hours 20 hours 40 hours

Note that air-cooled bikes should be inspected more frequently. Also, you may want to inspect more often if you are riding in fine sand or lots of mud. When you tear down the engine, inspect each system and look for the following trouble signs.

Reed Valve

Check the reed petals for open gaps between the sealing surfaces. In time, the reed petals lose their spring tension, and the back-flow can cause a flat-spot in the throttle response. Stock nylon reeds tend to split at the edges on bikes that are constantly over-revved. Expert riders find that carbon fiber reeds last much longer.

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Cylinder Head

Check the head at the edge of the chamber for erosion marks?a sign that the head gasket was leaking. If the head or top edge of the cylinder is eroded, it must be turned on a lathe to be resurfaced.

Cylinder

All cylinder bases use aligning (dowel. pins around two of the cylinder base studs. These pins are made of steel, and after heavy power washing, they get corroded. That makes it difficult to remove the cylinder from the crankcases. Never use a pry bar! That will damage the cylinder. Instead use a plastic mallet to hit upward on the sides of the cylinder at a 45-degree angle. Alternate from left to right sides so the cylinder lifts up evenly. After you remove the cylinder, stuff a shop towel into the open crankcases to prevent debris from entering the engine.

The Different Types of Steel-Lined and Plated Cylinders

There are two types of cylinder bores used on dirt bikes, steel or cast iron sleeves or ones with plating on the aluminum. Most dirt bikes made after 1989 have plated cylinders. You can check a cylinder with a magnet. If it sticks to the bore then it is a sleeve. If it doesn't stick then it is plated. There are three types of plated cylinders, Kawasaki Electrofusion, hard-chrome, and nickel silicon carbide. There are several variations of the nickel silicon carbide process but the most common trade name is Nikasil. The nickel-based processes have many advantages over hard-chrome, Electrofusion, and sleeving. Nickel attracts oil and is an excellent carrier material for silicon carbide particles, a wear resistant material that carries the load of the piston. This material is electro-plated right on to the aluminum cylinder for the optimum thermal efficiency. Nickel can be honed with diamond stones which leave distinctive peaks and valley scratches in the cylinder wall which retain oil and provide a certain bearing ratio between the running surfaces of the bore. It's possible to rebuild a plated cylinder by fitting it with a sleeve. However you can expect to pay more for bore maintenance over the life of the bike, and lose thermal efficiency and horsepower. Plated cylinders are harder and last longer than sleeved cylinders. Kawasaki cylinders with the original Electrofusion coating or hard-chromed cylinders can be repaired with nickel plating or sleeving. Steel or cast iron sleeves cannot be nickel plated unless they are separated from the aluminum cylinder. The reason is that the pretreatment for the plating would disintegrate the aluminum. There are four companies that replate cylinders in the USA. The average price to replate a cylinder is about $200.

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The Piston

Some unfortunate guys do more damage replacing the piston than the actual wear on the piston! Remove the circlips with a small needle-nose pliers and throw them away. It is a common mistake to reuse circlips, but the cheap spring-steel wire clips will fatigue and break if you install them for a second time.

After removing the circlips, you have to remove the piston pin. Never use a hammer and punch to remove the pin. That will damage the connecting rod and needle bearings. Instead, use one of the pin-extractor tools available from your local franchised motorcycle shop. You can also grasp the piston with one hand and use a 3/8-inch socket extension to push the pin out with your other hand.

Too many people replace their pistons too often. The exact service interval for your bike depends on how hard the bike was run, for how many hours, the quality of the lubrication, and the amount of dirt or other debris in the intake air. Bikes that are run hard with dirty air filters may wear out pistons in only 6 hours, while bikes that are ridden easy with clean filters and adequate fuel octane may last 60 hours.

Measuring the Piston

The best thing to do is measure the piston with a caliper. Digital calipers cost about $100 at industrial tool companies such as Enco or Harbor Freight. A digital caliper is easy to use and gives accurate measurements on the piston diameter and cylinder bore. Measure the widths of the piston (front to back) just above the intake cutaway because this is the widest point of the piston. Check the maximum wear specs in your service manual. Check the piston for detonation marks in the crown, cracks in the skirt, or seizure marks. Look at the underside of the piston crown for a large black spot. The spot is burnt oil deposits that adhered to the piston because the piston crown temperature was too hot. This is an indication that the carb's main jet needs to be richer.

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Letter Designations on Cylinders and Pistons

The Japanese manufacturers use a letter designation system for plated cylinders. They intend for you to order replacement pistons based on the letter designation printed or stamped on the cylinder. This is the reason why they need this type of system. In mass production you can't guaranty that all parts will be exactly the same size. The size variance is based on an acceptable level of quality. Tool bits become dull, temperatures of machine tools change through production runs, and machine operators have inconsistent performance. The Japanese manufacturers have between two to four different sized pistons and cylinders. Normally labeled A, B, C, and D. If they only had one size, the piston to cylinder wall clearance would vary between .001 to .006 inches. In the standard Japanese alpha labeling system, A denotes the smallest bore or piston size and every letter after that is slightly larger, usually in increments of .0015 inches. The danger is that if you try to put a D piston in an A cylinder the piston to cylinder wall clearance will be so tight that a seizure might occur.

Pro-X Oversize Piston Kits

Pro-X is a marketing company that sells the surplus pistons from the Japanese company ART, which makes all the cast pistons for the Japanese motorcycle manufacturers. These pistons are the same quality as the OEM pistons, and they are available in sizes larger than the alpha pistons available from franchised dealers. Also the Pro-X pistons are usually priced lower than the OEM pistons. If the cylinder bore is slightly worn (up to .005 inches) with only a small area of bare aluminum exposed, you can install a Pro-X oversize piston. The Pro-X pistons are graded oversize in smaller increments than Wiseco pistons, but a wider range than the OEM pistons. For example, Wiseco sizes are .010 inches and Pro-X is .001 inches increments. Before attempting to order a Pro-X piston, you must measure the cylinders bore at the smallest point and allow .002 inches clearance between the piston and cylinder.

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Measuring the Ring Gap

The best way to know if the rings are worn is to measure the ring end gap. Put the ring in the cylinder and use the piston to push it down about 1/2 inch from the top evenly spaced. Now use a feeler gauge to measure the width of the ring gap. Normally, the maximum gap is 0.018?0.025 inch.

Cylinder and Exhaust Valve Cleaning

Does your cylinder have burnt-on mud on the outside, heavy brown oil glazing on the cylinder bore, or gooey oil on the exhaust valves? If so, here is a tip for cleaning those parts without flammable cleaners. Go to the grocery store and get a can of aerosol oven cleaner. This stuff is great for cleaning the carbon from the exhaust valves without completely disassembling them. CAUTION: Oven cleaner attacks aluminum, so don't leave it on the cylinder for more than 20 minutes. Oven cleaner can be used on both steel and plated bores.

The oven cleaner will help loosen the oil glazing on the cylinder walls. Then, you can use a Scotch-Brite pad to hone the cylinder walls in a crisscross pattern. Wear rubber gloves when you use oven cleaner and flush the cylinder afterwards with soapy water. This will neutralize the acid in the oven cleaner and break the molecular bond of the oil, so the debris can be rinsed away. Sleeved (especially Kawasaki cylinder bores) are vulnerable to corrosion after cleaning. Spray some penetrating oil on the cylinder bore to prevent it from rusting.

Caution: Certain types of cylinders corrode quickly after the cleaning process, so spray the bore area with penetrating oil to displace the water.

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Honing the Cylinder Bore

Many people have emailed me with questions regarding honing cylinder bores. If you want to buy a hone to deglaze bores or polish off small scratches, then a ball-hone is the best choice. Ball hones are manufactured by Brush Research in Los Angeles, under the brand name Flex-Hone. These hones are available under different labels and they are most easily available from auto parts stores. Buy a size that is 10% smaller than the actual bore size. These hones are available in several different materials and grits but the profile that bests suits both steel and plated cylinders is aluminum oxide 240 grit. A ball hone cannot remove material from the cylinder bore, especially on the hard nickel plated bores. However a ball hone can polish down the peaks of the original hone scratches and increase the bearing ratio. In other words the piston will be touching a greater percentage of the bore. Sometimes that makes the piston wear quicker but if you have to ball hone the bore to remove scratches, it's a compromise. The one type of hone that you should never use on a two-stroke cylinder is a spring-loaded finger hone. The sharp edges of the stone will snag the port edges and most likely damage the hone and the cylinder.

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Top End Assembly

1. Install one of the circlips in the piston with the opening facing away in the 6 or 12 o'clock position.

2. Grease the cylinder-base alignment pins.

3. Set the exhaust valves in the closed position.

4. On cylinders with reed valves, leave the intake port open because you will need to reach in through the port to push the piston-ring ends back in place.

5. The best way to slip the piston into the bottom of the cylinder is to rotate the rings toward one side of the locating pins and squeeze the rings with your middle finger and thumb. That will leave your other hand free to position the cylinder.

6. There are two methods used to assemble to top end. The first method is to attach the piston to the connecting rod and lower the cylinder on to the piston assembly. The second method is to install the piston assembly into the cylinder and lower the cylinder and piston on to the connecting rod. The second method is easier but involves pinning the piston and installing one circlip with a minimum amount of free space.

7. Take care to align the exhaust valve control mechanism as the cylinder is bolted to the crankcases.

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Gasket Hygiene

The oven cleaner you used to clean the cylinders will help loosen the old gasket material so you can remove it. Carefully scrape the gasket off with a gasket scraper. Never use a flat screwdriver to remove the old gaskets because the aluminum surfaces of the head, cylinder, and crankcases are easily gouged. If these surfaces are gouged on your engine, they should be draw-filed flat to prevent air or coolant leaks.

Never reuse paper gaskets; always replace them with new gaskets, and spray sealer on the paper gaskets, so they will seal better and will be easier to remove the next time. The new-style steel gaskets can be cleaned and reused a few times, but you'll need to spray the gasket with a sealer such as Permatex Spray-A-Gasket or copper-coat.

Keep a Logbook

Keep a logbook that tracks the number of riding days and the periodic maintenance. From reviewing the log, you will learn how often you need to service the top end if you record the measurements of the ring gap and the piston diameter. A logbook also gives you greater leverage when you try to sell your used bike for a premium price.

Big Bore Kits

One of the best ways to increase horsepower is to increase displacement by over boring the cylinder. This can be ideal for play or Vet Class riders, where the increased displacement won't be illegal for your race class. When done right, a big bore kit can give you more power everywhere rather than an increase in only the top or the bottom of the powerband. Such increases are typically more usable and give you more power where you need it.

Piston manufacturers such as Wiseco make oversize piston kits for popular model bikes. These kits boost the displacement of the cylinder to the limit of a racing class or to a larger displacement class, for example: 80cc to 100cc, 125cc to 145cc, 250cc to 265cc or 300cc, and 495cc to 550cc.

The AMA has a limit of overboring any cylinder used in amateur modified classes. The limit is 2 millimeters. Wiseco makes a line of Pro-Lite pistons for this purpose. Normally no head modifications are needed, but cylinders with exhaust valves that operate close to the cylinder bore will need to be trimmed for clearance. Cylinders that use steel head gaskets will require oversize gaskets. Cometic makes 2 millimeter oversize and big bore gasket kits. The process of overboring and electro-plating a cylinder can be a cost effective way to save a cylinder that suffered a top end failure and scored the cylinder wall.

Riders competing in the AMA veteran class can ride a bike with any displacement. Riders competing in hare scrambles and enduro can race the 200cc class with a 125 converted to any displacement. AMA motocross and enduro racers can make the 250cc bikes legal for open class by increasing the displacement a minimum of 15 percent (to 286cc). Wiseco makes 74-millimeter piston kits to convert the popular 250s to 300cc. Be careful if you decide to go with a big bore kit, though. If the overbore is not performed properly, though, it can result in the wrong kind of power or, at worst, a ruined cylinder. When you change the displacement of the cylinder, there are so many factors to consider, such as port time-area, compression ratio, exhaust valves, carb jetting, silencer, and ignition timing. Here is an explanation of what you need to do when planning to overbore a cylinder.

Also, you should at least consult with an expert before tackling a big bore kit. To get the most from an overbored engine, you need to make sure the carburetion, exhaust, porting, and timing are all adjusted to suit the larger bore.

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Port-Time Area

The term port-time area refers to the size and flow range of the intake and exhaust ports, relative to rpm. The ports enter the cylinder bore at angles. When the cylinder is over-bored the transfer ports become lower and wider. The same thing happens to the exhaust port. This effectively retards the port timing and reduces the total degrees of duration. When the displacement of the engine increases, so does the demand for more port-time-area.

If you just overbored and plated a cylinder, it would have much more low-end power than stock but the top-end power would suffer. Normally tuners have to adjust the ports to suit the demands of the larger engine displacement. The proper dimensions for the ports can be calculated using a computer program from Two-Stroke Racing (TSR) http://www.tsrsoftware.com/ The program "PORTTIME" enables tuners with limited math skills to run strings of formulas for determining the optimum dimensions of the ports. Generally speaking, if the ports in the overbored cylinder were raised to the same heights as the stock cylinder, that would make the port timing sufficient to run with stock or aftermarket exhaust systems.

Cylinder Head

After overboring the cylinder, the head's dimensions must be changed to suit the larger piston. First, the head's bore must be enlarged to the finished bore size. Then, the squish-band deck height must be set to the proper installed squish clearance. The larger bore size will increase the squish turbulence, so the head's squish band may have to be narrowed. The volume of the head must be increased to suit the change in cylinder displacement. Otherwise, the engine will run flat at high rpm or ping in the midrange from detonation.

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Exhaust Valves

When the bore size is increased, the exhaust valve-to-piston clearance must be checked and adjusted. This pertains to the types of exhaust valves that operate within close proximity of the piston. If the exhaust valves aren't modified, the piston could strike the valves and cause serious engine damage. The normal clearance between the exhaust valves and the piston should be at least .030 inches or .75 millimeters

Carburetor

The larger the ratio between the piston's diameter and the carb's size, the higher the intake velocity. Overbored cylinders produce higher intake velocity which draws more fuel through the carb. Of course a larger engine will need more fuel. Normally when you overbore an engine 15-20%, the slow jet will need to be richened and the main jet will need to be leaned. Start with the stock jetting and make adjustments after you ride the bike.

Ignition Timing

The ignition timing has a minimal affect on the poweband. Retarding the timing has the affect of reducing the hit of the powerband in the midrange and extending the top end over rev. "Overrev" is a slang term that describes the useable length of the powerband at high rpm.

The scientific reason for the shift of the powerband to extremely high rpm, is because the temperature in the pipe increases with the retarded timing, and that enables the pipe's tuned length to be more synchronous with the piston speed and port timing of the cylinder.

Advancing the timing has the affect of increasing the midrange hit of the powerband, but makes the power flatten out at high rpm. The reason is that the relatively long spark lead time enables for a greater pressure rise in the cylinder before the piston reaches TDC. This produces more torque in the midrange but the high pressure contributes to pumping losses at extremly high rpm.

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Pipe and Silencer

Because only the bore size is changed, you won't need a longer pipe?only one with a larger center section. FMF's line of Fatty pipes work great on engines that have been overbored.

Head Gasket

The head gasket will need to have the bore diameter increased to the dimension of the new piston. If the head gasket overlaps into the cylinder bore more than one millimeters on each side, it could contact the piston or be susceptible to pressure blowouts.

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10 TIPS FOR REBUILDING A TWO-STROKE TOP END

1) Before you disassemble your engine, power-wash the engine and the rest of the vehicle. That will reduce the risk of dirt and debris falling into the engine. Once you remove the cylinder, stuff a clean rag down into the crankcases.

2) The cylinder and head use alignment pins to hold them straight in position from the crankcases on up. The pins make it difficult to remove the cylinder from the cases and the head from the cylinder. Sometimes the steel alignment pins corrode into the aluminum engine components. Try spraying penetrating-oil down the mounting studs before attempting to remove the cylinder and head. Never use a flat-blade screwdriver, chisel, or metal hammer to remove the cylinder. Instead use this technique; buy a lead-shot plastic mallet, swing it at a 45-degree angle upwards against the sides of the cylinder. Alternate from left to right, hitting the sides of the cylinder to separate it from the cases evenly. Clean the steel alignment pins with steel wool and penetrating-oil. Examine the pins closely. If they are deformed in shape, they won't allow the engine parts to bolt together tightly. This can cause a dangerous air leak or a coolant leak. The pins are cheap at about $2 each. Replace them if they're rusty or deformed.

3) Never re-use old gaskets. Remove them with a razor blade or gasket scraper. Don't use a drill-driven steel wool type pad to remove old gaskets because they can remove aluminum from the cylinder and head. That will cause a gasket to leak.

4) Always check the ring end gap on a new ring by placing it in the cylinder between the head gasket surface and the exhaust port. The gap should be between .012 to .024 inches.

5) Always install the circlips with the opening facing straight up or down, that way inertia will hold it tight into the clip groove. Place one clip in the groove before installing the piston on the connecting rod. Its easier to install a clip with the piston in your hand rather than on the rod. There also less chance that you'll drop the circlip in the crankcases.

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5) Always install the rings on the piston with the markings facing up. Coat the rings with pre-mix oil so they can slide in the groove when trying to install the piston in the cylinder.

6) Always install the piston on the connecting rod with the arrow on the piston crown facing towards the exhaust port.

7) The traditional way to assemble the top end is to install the piston assembly on the connecting rod, compress the rings, and slide the cylinder over the piston. That can be difficult with larger bore cylinders, or if you're working by yourself. Try this method instead. Install one circlip in the piston, install the piston into the cylinder with the pin hole exposed, install the piston pin through one side of the piston, position the cylinder over the connecting rod and push the piston pin through until it bottoms against the circlip, install the other circlip. It only takes two hands to install the top end using this manor and there is less chance that you'll damage the rings by twisting the cylinder upon installation.

8) On cylinders with reed valves and large oval intake ports, take care when installing the piston assembly in the cylinder because the rings are likely to squeeze out of the ring grooves. Use a flat-blade screwdriver to gently push the rings back in the grooves so the piston assembly can pass by the intake port.

9) For steel head gaskets, place the round side of the "bump" facing up. Don't use liquid gasket sealer; use aerosol spray adhesive types instead. For hybrid fiber/steel ring head gaskets, place the wide side of the steel rings facing down.

10) When you initially start the engine after a rebuild, manipulate the choke to keep the engine rpm relatively low. Once the engine is warm enough to take it off choke, drive the vehicle around on flat hard ground. Keep it under 2/3 throttle for the first 30 minutes. Two common myths for proper engine break-in are; 1) Set the engine at a fast idle, stationary on a stand. 2) Add extra pre-mix oil to the fuel. When the engine is on a stand it doesn't have any air passing through the radiator and it is in danger of running too hot. When you add extra oil to the fuel you are effectively leaning the carb jetting. This can make the engine run hotter and seize.

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Top End FAQs

Thin Sleeve Causing Seizures

Question: My 1987 CR125 has chronic piston seizure problems. The cylinder is bored one millimeters oversize. The lower end was rebuilt so I know it doesn't have a crankcase air leak. What could the problem be?

Answer: The original cylinder for your model bike had a very thin steel sleeve. Honda only offers one oversize piston. When the sleeve is overbored too far, the sleeve cannot transfer out heat into the water jacket efficiently. The heat builds up over the exhaust port, and the piston melts. You have two repair options: buy a new cylinder or install a new thicker sleeve in the old cylinder. Wiseco offers thick sleeves and forged piston kits.

Honda CR250 1988?91 HPP Problems

Question: My 1990 Honda CR250 is making me wacky. I tried to check the exhaust valve system, and I don't think it works properly. I removed the left-side valve cover from the cylinder, revved the engine and the valves hardly moved. They don't open fully when the engine is revved, and they don't close completely either. What is the most common cause of this problem and how can I fix it myself?

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Answer: The problem is that the HPP mechanism isn't fully engaged, and the valves are just moving from the exhaust-gas pressure. The most common problem with the 1988?91 CR250 HPP systems, is the improper engagement of the governor control and the spindle rod that actuates the HPP valves. The following procedure may cure the problem. Remove the top right valve cover on the cylinder and the round-slotted access cover located under the water pump on the right side engine cover. Insert an 8mm T-handle through the access hole and onto the detent bolt that locks the governor control to the cam spindle, and turn the bolt 1/4 turn counterclockwise. Now, the bolt has disengaged the HPP system. Insert a straight-blade screwdriver into the slot in the top of the right-side pinion shaft (from the top right side of cylinder). Turn the pinion shaft counterclockwise 1/8 turn, and then turn the detent bolt (located under the right-side engine cover) 1/4 turn clockwise. It is important to release the spring tension from the pinion shafts in the cylinder to engage the detent bolt. This procedure also enables the HPP mechanism to be engaged without any chance of damage occurring to the fragile cam spindle.

Top-End Big Bore

Question: I have an old cylinder for my 250. The bore was ruined when the head gasket leaked, and there is severe erosion on the top edge of the cylinder. I read your article on top-end rebuilding and had an idea and a related question. I compete in amateur enduro events and the rules state that the displacement of bikes competing in the open class must be a minimum of 251cc. My question is, can I salvage this old junk cylinder by overboring the cylinder to fit a Wiseco piston kit and have the bore re-plated? If yes, will my bike be legal for the open class?

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Answer: There are a number of companies offering cylinder repair services and replating. The way to fix the erosion problem is to heli-arc weld aluminum over the erosion and then re-face and bore the cylinder. WISECO and L.A. Sleeve make oversize piston kits and gaskets for most Japanese dirt bikes. The common overbore displacement sizes for 250s are 265, 285, and 310cc. After the cylinder is re-plated, the exhaust valves and the cylinder head must be matched to the larger bore size. This involves special metal machining and should be trusted to a qualified tuner or machinist. This type of mod will enable you to race your 250 in the open class.

Kawasaki Air/Oil Leaks

Question: My son and I are just getting started in dirt-biking. Over the winter I bought him a 1989 KX80 as a basket case. We are learning about dirt bike repairs by rebuilding this bike. It's a lot like model building, only the parts are old and greasy! We inspected the crankcases and noticed that there was some oil leaking from the three oval-shaped plugs that are spaced an equal distance around the main bearings. How can we repair this problem without buying new crankcases?

Answer: Every Kawasaki dealer's service department has a Team Green book with tips on how to repair common problems. Ask your dealer's service manager for a copy of the Team Green bulletin. It has photos and drawings of how to apply the epoxy over the crankcase plugs.

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Top-End Seized After Rebuild

Question: I trail ride a 1989 YZ250. Last winter, I rebuilt the top end after reading your article in Dirt Rider. The bore was so worn that I had to skip to a one millimeter-oversize piston kit, just so the bore job would clean up a severely worn spot below the intake port. After I rebuilt the top end, I cycled the engine by letting it idle for three 15-minute sessions with adequate cool-down periods in between. When I first rode the bike, I heard some detonation noises but didn't think it was a serious problem, until it seized. What could be wrong?

Answer: Your problem is simple. When a cylinder is overbored, the displacement is increased and that boosts the compression ratio. Whenever a cylinder is overbored more than 0.010 inches or 0.25mms the cylinder-head diameter must be enlarged to the new bore size. Otherwise, the piston could contact the head or the edge of the head surface that extends into the bore could cause a hot-spot and pre-ignition. Also, the cylinder head's squish band must be narrowed by enlarging the combustion-chamber bowl. This also serves to increase the head's volume, thereby lowering the compression ratio. This work must be performed on a lathe by a qualified tuner or machinist. Average cost of this service is $50

Base Gasket Seeping

Question: I recently rebuilt the top end on my 1991 CR250. I was being as careful as I could be while taking the cylinder off, but the dowels were fused in pretty good and I had to pry it. Needless to say, I gouged the case a bit. I smoothed it out with sandpaper and reassembled the engine. The bike runs great, but a little oil seeps out of the cylinder-to-case mating surface. I assume this is transmission oil? Would it be OK to use something like a thin layer of Permatex Blue or Yamabond here? Would this make it even more difficult to remove the cylinder in the future? Should I just let it alone? The best price I could find on a new left side case was $215 and I'm sure it would be a lot of work and a lot of replacing gaskets along the way. Am I out of luck?

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Answer: Air leaks can be very dangerous because the engine could rev independent of the throttle. An inexpensive way to fix your bike's problem is to draw-file the cylinder base and the crankcases. Then apply a thin coating of Yamabond or any other brand of non-drying sealer to both sides of the base gasket. The best technique for removing cylinders is to tap up on each side of the cylinder with a lead-shot plastic mallet. Remember to put a dab of grease on the cylinder-base dowel pins.

Frequency of Top-End Rebuilding?

Question: I have a 1990 RT180, and I don't think the rings or piston have been replaced. I don't know if the top end has ever been rebuilt because I bought the bike used. How long do piston and rings usually last on a two-stroke engine like mine? How often should the piston and rings be replaced, and should I replace them now?

Answer: Replace the piston and rings before they wear out. The time scale varies between models, usage, and preventive maintenance. The only way to determine the condition of your bike's top end is to disassemble the top end and measure the piston diameter and the ring end gap. Compare the measurement to the maximum wear specs published in the service manual.

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 Two-Stroke Exhaust Valves

Three words sum up exhaust valve maintenance: spoogey, gooey, and grungy. If two-stroke exhaust valves didn't have such a dramatic effect on the engine's powerband, I'm sure mechanics would remove them and beat them bits with a hammer in frustration because there is little information given by the manufacturers on how to diagnose and repair the exhaust valve systems on well-used dirt bikes. This section is a guide to the characteristic mechanical problems that occur to the exhaust valve systems of dirt bikes. Plus we'll give you some tips on how to re-time exhaust valve systems.

How Exhaust Valves Work

An exhaust valve system is designed to increase the engine's low-end and midrange power. There are three different designs of exhaust valve systems. The first-generation design uses a variable-volume chamber mounted to the head pipe to change the tuned length of the head pipe. A butterfly valve is used to separate the surge chamber and the head pipe. At low rpm, the valve is open to allow the pressure waves in the pipe to travel into the surge chamber and effectively lengthen the pipe and reduce the pressure wave's magnitude when it returns to the exhaust port. These systems were primitive and not very effective on 125cc dirt bikes. Honda and Suzuki used this type of exhaust valve system in the mid to late 1980s.

The second-generation design features valves that control the effective stroke and the time-area of the exhaust port. These valves are fitted to the sub-exhaust ports and the main exhaust port. The main exhaust-port valves operate within close proximity to the piston to control the effective stroke of the engine. The effective stroke is defined as the distance from TDC to when the exhaust port opens. At low rpm, the engine needs a long effective stroke, which results in a high compression ratio. At high rpm, the engine needs a shorter effective stroke, longer exhaust duration, greater time-area, and a lower compression ratio. Yamaha used this system starting in 1982 on the YZ250. Honda's HPP system is similar and was used on the 1986?91 CR250 and 1990 to current-model CR125.

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The third generation of exhaust valve systems attempts to change the exhaust-port velocity, effective stroke, exhaust-gas temperature, and the pressure of the compression wave. Yamaha and Suzuki started using these systems on their 125s in 1995. Both companies employed a venting system to the outside atmosphere. This is very complex because they are attempting to affect the temperature and pressure of the returning compression wave to synchronize it with the piston speed. The exhaust-gas velocity and the effective stroke are controlled by two oval wedge valves that enter the exhaust port at a 45-degree angle. The wedge valves partially block the exhaust port, thereby boosting the gas velocity. Kawasaki's KIPS system uses wedge valves in the main exhaust port to control the effective stroke, drum valves in the sub-exhaust ports to control the time-area, and a surge chamber to absorb the excess compression-wave pressure at low rpm.

The exhaust valves are opened and closed by a centrifugal governor mechanism. The governor is mounted under the right side cover and is gear-driven by the crankshaft. As the engine rpm increases, the governor spins, thereby increasing the angular momentum of the four steel balls encased in the governor. The steel balls fit into an angled ramp-and-cup arrangement. A spring is used to provide tension on the steel balls. When the momentum of the steel balls overcomes the spring's tension, and the balls force their way up the angled ramp. A spool attached to the ramp, enabling it to change its linear position with changes in rpm, and the spool is attached to a linkage system that operates the exhaust valves in the cylinder. Factory race teams have different combinations of springs, ramps, and balls to tune the exhaust valve operation and enhance the powerband.

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Exhaust Valve Tips and Tuning

Although exhaust valves use the same essential principles, the implementation is different with each manufacturer. Also, each type has its own flaws and fixes. The list below gives you tips on how to install and service the most common exhaust valves, as well as some tuning tips

Honda HPP

Honda's HPP system started as a butterfly operated canister mounted between the cylinder and pipe. It served to control the volume and length of the exhaust pipe. It had little effect on the power and most aftermarket pipes eliminated the canister. The butterfly was prone to carbon seizure and required frequent maintenance. The next generation HPP was used on the 1986-91 CR250. This system featured two sliding valves that operated within close proximity of the piston and effectively varied the exhaust port time-area in accordance with rpm. The square valves moved horizontally through a valve guide. The system was plagued with a mixture of design problems and misinformation on how to service and re-time this complicated exhaust valve arrangement. This section lists some common problems and some tips for timing the system, installing the cylinder, and engaging the HPP mechanism.

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Common HPP Problems

Two main problems plague the HPP system: carbon fouling and rack-and-cam-spindle damage. The square shape of the valves contributes to the accumulation of carbon on one corner of the valve guide (stationary part), in the corner of the guide that is directly in the exhaust gas stream and this causes the valve to become carbon seized. Chamfering the corresponding edge (one-millimeter) of the valve will eliminate this problem. The rack and cam spindles are easily damaged when the cylinder is installed incorrectly, or the HPP mechanism is engaged incorrectly. See the photos for examples of damaged rack and cam spindle parts.

HPP Timing Procedure

Use the following procedure to time the HPP system:

1. Install the HPP valves and levers and tighten the pivot nuts. Place the washer on the stud first, then the lever (marked left and right), and then the flanged center bushing with the flange side facing up.

2. Turn the cylinder upside down. To position the rack correctly, slide it to the left until it stops; then move it right 2mm. Rotate the rack so the square notch faces you. Now the rack is in the correct position so you can install the pinion shafts. Carefully turn the cylinder right side up without changing the position of the rack.

3. Close the valves and install the left pinion shaft with the screwdriver slot facing the one o'clock position. Install the right pinion shaft with the screwdriver slot facing the eleven o'clock position (see photo for correct positions). A simple way to determine if the pinions are mis-timed to the rack is to look at the screwdriver slots. The wrong position is with both slots facing twelve o'clock.

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Installing the Cylinder and Engaging the HPP Drive

After timing the HPP mechanism, the cylinder is ready to be installed on the crankcases. Here are some tips for installing the cylinder and engaging the HPP drive mechanism:

1. Make sure the reed valve is removed from the cylinder. CR250s have such large intake ports that the rings tend to slip out of the ring grooves during installation of the cylinder. This takes the spring pressure off the cam spindle. Now turn the engagement bolt 1/4 turn clockwise. You should feel it positively lock into a groove and stop. Remember that the HPP engagement bolt is a spring-loaded detent not a threaded bolt. Slide the cylinder down onto the piston and rings, use a screwdriver to push the rings back in the grooves until the rings clear the intake port.

2. The HPP mechanism should be engaged while the cylinder is being installed, just to keep the cam spindle in position. The cylinder will stop about 3mm from the crankcases because the cam spindle and the rack are misaligned. Now disengage the HPP mechanism by turning the engage bolt 1/4 turn counter-clockwise. Grasp the right-side valve lever and wiggle it; the cylinder should then drop evenly onto the crankcases.

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3. Bolt the cylinder down tight. The best way to engage the HPP mechanism is to insert a screwdriver in the right-side pinion shaft and turn it counterclockwise. Now turn the engagement bolt clockwise. You should feel the engagement bolt lock positively in position. If you try to rotate it too far, you will bend the cam spindle and the system won't work at all, so don't be a hammer-head! The best way to check the HPP system is to remove the left-side valve cover from the cylinder, start the engine and warm it up, then rev the engine. The valves should be fully closed at idle and fully open when the engine is revved.

In 1992 Honda introduced the HPP system currently used on the CR250. This system features a center valve for the main exhaust port and two rotating drum valves to control the flow of the sub exhaust ports. This system also features a return of the old resonator as used on the mid-eighties model. The resonator improves the throttle response and mellows the powerband at low rpm. A thin rod links the valves together and the whole system is mostly self-scraping to prevent carbon build-up. The inside of the center valve has an elongated passage where the tie rod travels. This elongated passage is prone to carbon build-up over time (1-2 years). The carbon limits the range of movement in the valves. The carbon is easily removed by using a small diameter rat-tail file. The sides of the center valve and the drum valves interface, and that area is prone to carbon build-up too. A wire brush or file is an effective tool in cleaning the exhaust valves. Here is a simple way to check the operation of this system. On the left side of the cylinder there is a 17mm cap bolt that exposes a straight line mark in the left drum valve. There is a corresponding mark on the cylinder. The "L" mark denotes the low speed position of the valve and the "H" denotes the high-speed position. To check the HPP, start the engine. At idle the valve should align with the "L" mark. Then rev the engine, the valve should align with the "H" mark. If the angle of the mark on the valve is slightly off, then the valve probably needs to be de-carboned. This system is very easy to disassemble and can only fit together one obvious way so we won't waste space on that procedure. There is some aftermarket parts to adjust the performance of this system for different types of dirt biking. Pro-Racing in England makes a spacer for the right side valve cover. It serves to add volume and length to the resonator part of the system. This is especially suited for enduro riding where a smooth transition to the mid-range is important for better traction. ESR (Eddie Sanders Racing) in California makes a replacement HPP system that holds the valves wide-open. The center exhaust valve is thinner which enables tuners to raise the exhaust port. The ESR system is primarily used for dirt track or kart applications where low-end power is of no consequence.

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Whenever the cylinder is installed on the bottom end after top end rebuilding, the valves need to be put in the closed position. Otherwise the HPP cam spindle that connects the actuator in the cases to the cylinder will get damaged when you tighten down the cylinder. That will also make the valves inoperable. Always check the HPP valve operation after you assemble the top end by using the inspection cap on the left side of the cylinder.

The CR125 HPP system was redesigned in 1990. Honda chose to use a system similar to the 1986-91 CR250, featuring horizontally sliding valves. This system was plagued with problems over the years. The valves are prone to carbon seizure because the critical square edges face the exhaust stream. If the clips that fit on the ends of the valves vibrate off or if the valve wears too much then the valve can tilt on an angle and strike the piston. Another common related problem happen when tuners widen the exhaust port during porting and neglect to grind the valves at the outer corners for piston clearance. There again the piston strikes the valves because they protrude into the bore. In 1998 Honda made a modification to the valves, they added an L-shaped rib that prevented the valves from angling in and contacting the piston. The other problem of clearance between the top of the valve and the guide was eliminated so the new style valves provide more low-end power. These valve and guide sets from the 1998-99 models fit the CR125 models back to 1990.

In 2000 Honda redesigned the CR125 engine and adapted the exhaust valve system used on the RS250 roadracer. Honda also used this system on several dual sport and street bikes sold in Asia and Europe. The new system is so simple and effect

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