Maintenance & Technical > KX500 Steel Frame Conversion (SFC)
2003-2007 Steel Frame Cookbook
umberto:
Step 5 - put the motor in the frame and modify the swingarm
Once you have cut the front motor mounts and head stay off, you can fit the motor to the chassis. Be aware that some years of K5 use smaller motor mount bolts than the 250, so you may have to use the 500 bolts or head to your local Fastenal to get the motor secured. The bottom motor mount and the swingarm pivot will locate the motor in the chassis, allowing you to measure and cut the swingarm to fit the more narrow opening. You should have to remove material equally from each side of the swingarm in order for it to fit. Remove the races from the swingarm before you cut, as they are hardened and will not cut with a normal bandsaw blade. You will trash the installed bearings, but you already purchased a new set of races and bearings that you can use once you have the swingarm adjusted to fit. Remember that you will need a little extra room for the lip of the race, so measure the race thickness before you start cutting. I cut my swingarm on a bandsaw a little large, and then slowly adjusted with a belt sander. Remember you will need to cut the races down as well so that they fit in the narrower opening. I wrapped mine in tape until the tape was level with the lip of the race so I could cut it with my carbide cut off saw. Once you get the swingarm fitting, cut your new races and install the new bearings. You're getting closer.
As we talked about in Step 4, now is a good time to fab your mounts and weld them up. You will need to do a little more welding, but the mounts are the major part.
umberto:
Step 6 - Radiators
The radiators will get snug very quickly. In order to make more room, you will need to weld up the bottom mounting hole and move the threads as far forward as you can. You won't gain a ton of space, but it will help. Put your radiators on with the transfer hoses attached, and slide them as far forward as you can at the bottom without pinching the hose off. Mark that location and either drill and tap the hole, or use a Rivnut thread insert. You can use a combination of the stock 250 hoses and the 500 hoses to run your radiator lines. Don't be afraid of using a hard tube to join the two together, just make sure it's aluminum as two different metals can cause corrosion and clog your radiators. Always put a little extra past what you need to keep the hose from popping off.
On my conversion, I used a tube on one side and a temperature sensor for my Trail Tech Vapor on the other. I've had my together for two years as I write this, with no issues.
umberto:
Step 7 - Airbox
The most critical step to this rather easy process is to have the right parts. As I mentioned earlier, you will need the KDX intake to aim your carburetor in the correct direction. I used a 1996 KX 500 airboot and filter parts (cage, mounting rings, etc.). The most important part is to have the 2004-2007 airbox, which will bolt in to the 2003 subframe without any issues. The reason you need this airbox is because it is stepped back just enough to allow you to mount the KX 500 airboot without compressing it (see picture one below). You will need to make an aluminum plate to go over the entire front of the airbox. I used the securing ring from the 250 and added 1" all around to cover the hole (see picture two below). Install that on the existing airbox with some RTV and rivets. Install your intake, carb, airboot, and airbox on your bike. Mark as many of the airboot holes on the aluminum as you can. Take everything off and put the airboot on the airbox. Mark the final holes and use the mounting ring to trace the hole. Cut that hole out and mount the KX 500 airbox parts in the 250 airbox.
umberto:
Airboot clearance
umberto:
Step 8 - Pipe
Of all the steps, this one is the most difficult or simple. It's simple if you can purchase a Service Honda (no longer being made, but available on eBay), DynoPort, or any other pipe made for conversions. They are bit more expensive, but may be a break even when you consider the price of buying multiple pipes to craft together, welding time, frustration, years off your life, etc. If for some reason we are in an apocalyptic future where these pipes are no longer available, then your only option is combining a KX 500 pipe with a CR 500 pipe. This sounds simple, but be aware that the CR pipe hugs very tightly to the KX engine and will need lots of hammer adjustments to make it fit. Be prepared to do mean things to your pipe. Your main area of interference is the coolant transfer from the water pump to the cylinder head. You will need to dent the pipe pretty heavily in this area. I have seen people use a trick Service Honda employed where they flip the fitting and use a special hose to come in from the top. You will need to do search of the site to find this trick.
When you get your pipe finished, do yourself a favor and spend some time on the mounting bracket. It will need to be beefy to handle all the vibration. While you have the motor apart or the electronics safely out of the motor, you have the opportunity to weld on some beef to the mount. It doesn't need to support a tank's weight, but it does need to be beefier than the 250 mount. You don't want your pipe coming off. I used some 3/16" plate and a solid rod that I threaded to accept the 03 250 half rubber mount. So far my pipe is still on.
My pipe was a KX 500 inlet, and parts from a trashed CR pipe and a good CR pipe. I used the trashed CR pipe to add material to push the pipe out near the interference areas (ignition cover, transfer pipe). I added about 1 1/2" in the front bottom, and a 1 1/2" to 1/2" wedge in the front right above the engine exhaust port (the thickest part faced mostly forward). The wedge turned the pipe out a little bit as it came around, giving me a little more space in the transfer pipe area. Even with all that I still had to dent my pipe. This is a trial and error part, and will require some decent fabrication skills. Did I mention that people make pipes for this conversion?
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