Maintenance & Technical > KX500 Steel Frame Conversion (SFC)

2003-2007 Steel Frame Cookbook

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umberto:
I wanted to summarize the steps I took to build my 2003 SF conversion, so you could build your own bike without wading through my long thread.  I boiled it down to the essentials for building a bike.  If you have any questions or would like to see more details, please let me know at the bottom.

I'd like to start by thanking zz3gmc for all his advice on the specifics of doing this conversion.  He's the one who figured out all the parts you need to get around the challenges.  Many thanks also to Sandblaster, Dave916, DoldGuy, Danger4U2, KXDino and many other members for their assistance with stupid questions, borrowed parts, replacement parts, and field testing.  If you don't know, there are some very amazing people on this site who will help you with anything.  As you start your conversion, get on this site and post your progress.  We love to follow a build, and love to help out with questions.

umberto:
Step one: picking your bike

The 2003-2007 250s are fairly easy to convert to a 500, and make a very fun ride if you pick the correct year.  The 2003-4 are very similar, with the only difference being the front brake routing, rear suspension linkage, and a few motor differences.  The 2005-7 are very similar as well. 

Which bike to pick is a hotly debated question.  I've had both year ranges, and I prefer the 2003-4, because they were the last front end steering two stroke 250s.  I would say the 03 is the better choice, because they handle just a little bit sharper than the 04 in my opinion.

As for the 2005-07, KXDino has commented on this site that they killed those bike's handling by steepening the rake by a full degree (from 27 degrees to 26).  They can be evil handling bikes, and the only cure that KXDino has found is to relax the steering head angle a half degree to 26.5.  If you are comfortable with doing that, then the later bikes could be just as much fun to ride.

umberto:
Step two: What do I need?

You will need the following list of stuff to convert your bike to a 500.

Donor 250 - Sell the motor, electronics, pipe, and silencer
500 motor, electronics, pipe
Service Honda conversion pipe (search on eBay) OR
KX 500 pipe, CR 500 Pipe to splice together
CR 500 silencer from a later model bike (2000s)
If you using a 2003 KX 250, you will need a 2004-2007 airbox (more on that later)
KDX 250 intake boot.  It fits the 500 bolt pattern and turns the carb to go around the shock
Later model (89 and up) KX 500 airboot and airbox internals
1993-1998 KX 125 Carb top.  It allows the cable to bend and fit under the tank
1993-1998 KX 125 throttle cable
KX500 clutch cable
One set of swingarm bearings - you will be cutting the swingarm, and the old bearings will get full of debris
Metal to make the front motor mounts
aluminum to put on the airbox and mount your filter

umberto:
Step Three - drill the swingarm pivot

Before you start fabrication, take a moment to pull off the ignition, carb, and any coolant fittings you can get off.  Seal those holes with plates or tape to keep debris out.  Pulling your electronics will allow you to weld on the chassis with the motor in it, without fear of frying them.

The KX500 uses a smaller swingarm pivot than the 250s and later aluminum framed bikes.  You will need to drill the cases on your KX 500 in order to even get it in to the frame.  The good news is that once your drill the cases, the lower motor mounts line right up and allow you to mount the motor in the frame where it needs to be while you make your other motor mounts.  The not as good news is that the head stay and front motor mounts will need to be redone.  

There are two ways to drill the motor cases to fit the larger pivot bolt.  The preferred method is to drill each case separately while the motor is apart, but obviously this is not possible if your motor is already together.  If your motor is together, you can build a jig to hold the motor perfectly level and drill both sides at once.  Make sure you motor is level in every direction, and take your time.  You can really screw up cases if you get off center.  I built a jig using plywood and all thread that allowed me to drill the motor in one piece.  I attached a picture below.

The drilling of the cases will need to be done with a 43/64 (17mm) drill bit.  You will only be drilling a little bit on each case at the edge, not all the way through, so it goes fairly quickly.  Be sure to use plenty of lubricant and check your level every chance you get.

umberto:
Step Four - cut the front motor mounts and head stay off

In order to fit the engine in the frame, you will need to cut off the front motor mounts and fabricate new ones.  You will also need to cut the head stay mount loose from the frame.  It will need to be centered in the frame (it is offset to the left side of the bike from the factory) for gas tank clearance.  Be careful when cutting and don't nick the frame.  It is better to leave a part of the motor mount on the frame and grind it off, than weakening the frame.

I fabricated my motor mounts like Kawasaki did: in two parts that meet at the motor.  In fact I made cardboard templates based off the stock motor mounts that I cut off.  I attached a picture below.

The head stay is pretty simple.  Just straighten the 250 head stay aluminum brackets and tighten them to the head.  Straightening the head stays brackets will center them in the frame.  Measure how far it is back to the frame and cut your head stay frame piece that you removed earlier to that length.  Weld it all around and you're good to go.

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