2003-2007 Steel Frame Cookbook by Umberto

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I wanted to summarize the steps I took to build my 2003 SF conversion, so you could build you 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.

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.

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

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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.

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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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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.

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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.


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 has 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.

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Airboot clearance

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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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Step 9 - Mounting the Silencer

If you are reading this, you made it through the pipe. Congratulations. No really, that was the hardest part. Mounting the silencer is very easy. Just put your CR pipe seal on the pipe you just finished or hopefully purchased, and slide the silencer on the pipe. Make two brackets from aluminum that tie the mounting holes on the silencer to the mounting tabs on your subframe. Don't have two mounting tabs on your subframe? Get some thicker aluminum and fabricate one for your welder to add. You will need two mounting holes for this beast. It will vibrate!


Step 10 - Electronics

This is the step to add back your electronics that you took off before fabrication began. There isn't a lot to a 500's electronics, so you are in for a bit easier step than if you were wiring a 250. There are a couple of things to keep in mind. You will need to lengthen the wires coming from the motor to the CDI box. I mounted my CDI box on the left side of the bike in the hole where the 250 had its resistor. I made a tab to mount my coil on the right side of the frame behind the radiator (see picture below). I initially put it in front of the motor, but it kept frying coils. The new location is safer and has lasted a year and a half. Once you have everything mounted, you will need to solder extensions to the middle of the motor wires. Be sure to use heat shrink tubing over the solder connections, and consider a longer heat shrink to hold the wires coming from the engine. This will protect the wires from the elements and wear and tear.

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Step 11 - Controls

Alright, you are getting close to done. Now we just need to do controls. I used the 250 throttle assembly, 1996 KX 125 cable, and 1996 KX 125 carb top. The clutch cable was a KX 500 cable that I bent slightly to follow the stock 250 routing. Nothing special here, just having the right parts.

Step 12 - Gas tank

Now you are really close to getting finished. All you need is the gas tank and you are ready to ride this beast. In step four, we talked about centering the head stay in the chassis. This step is critical in fitting a gas tank. I'm going to assume that you are like every other 500 rider, and want as much fuel on board as possible. Thankfully there is a solution that gets you very close to 3 gallons of fuel. Not enough for Baja, but plenty for normal riding. The key to the gas tank is buying the right tank. You must buy the Clarke 2007 KX250 tank. No other tank will work! I know I tried them all for you. I bought mine in black from justgastanks.com. Before you start, remove the little rubber tank mounting strips from your frame. The oversize tanks usually don't use them. Mounting the tank may require you to break out the heat gun and do a little forming, but it shouldn't be too bad. Just remember to go slowly and don't keep the heat gun on your tank for too long. Start slowly and you should get the hang of it. You will need to fabricate a new front mounting hole, as the old one will be about 1/2-3/4" too far back to get the tank to fit. I used a rivnut instead of a bracket, and after I had installed it, the lip of the rivnut touched the top of the old mounting hole.

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Step 13 - so riding isn't 13 - Final Prep

Check all your bolts, connections, fuel lines, EVERYTHING! This dude will vibrate, and you don't want anything coming off. Check it twice just to be sure. This bike will be fast, and you don't want to end up in the hospital. Don't ask me how I know this.



Step 14 - Ride that dude

Okay, now you are ready to ride. I would suggest riding near the truck, and keeping lots of tools with you. It will have issues, no matter how hard you try to prevent them. One thing I would always carry with me are the tools to take the gas tank off. You will not be able to get to the plug without taking off the tank. Unless you like to push, I would put a ratchet, sockets, and a pipe puller (for the tank strap), by your extra plugs in your tool pouch.

Now, go out and ride that dude. Have your buddy film you so we can watch.