More on Exhaust

Now it is time to mount the silencer. Some silencers may come very close to mounting on the stock mounting brackets, with some trimming of the rear fender plastic. Certainly, with a bracket you could make it work without modifying the sub-frame. But since I don 't want my bike to look like a science project, I will add new brackets for the silencer.

Start by removing the old lower silencer bracket. In general when cutting off welded components, you don't want to cut through the weld, rather cut on the side of the weld away from the part that you want to keep (as marked in blue). We can discard the little tab, but we don't wan't the tube to look hacked up.

This way, when you break the tab off, it won't take a chunk of the tube with it; then you can carefully sand away the weld with a sanding wheel on your angle grinder.

I made some aluminum tabs out of 5/16 aluminum plate. You can buy stainless steel press-in nuts from McMaster-Carr to press into the hole in the tab. They can be pressed into the hole with a vice or even just hammered in. These are much better than just using a nut on the other side because the don't rattle loose as easy, and they match all the other threaded inserts on your sub-frame.

Make the tabs large-ish because they will be much easier to weld if they are bigger/thicker (and they will be less likely to break off). Be sure to have the motor in the bike, the rear fender mounted and the expansion chamber on when you are deciding precisely where to mount the silencer. You will have to cut the tail of the expansion chamber off to get it out of the way while you are positioning the silencer (it has to be shortened anyway).

Position the silencer as far back as possible so you dont have to shorten the expansion chamber much, as it can have an effect on performance. I welded the upper tab on to the bottom of the original upper mounting tab and ground them to look like a single part. the upper tab will still hold the fender. The lower tab actually needed to be inward a bit (on bottom of the tube rather than on the outside) to get proper alignment where the pipe and silencer meet (Be sure to put on the plastic side plate when deciding where to position the silencer.)

You should shoot for about a 1/2" gap between your silencer and the rear fender to prevent the fender from melting.

Position the silencer such that the inlet is only about 1/4" away from the frame to leave ample clearance for the shock. Line up the tail of the expansion chamber with the inlet of the silencer and make sure that the expansion chamber doesn't interfere with the kick-starter, frame, carb or coolant tubes entering/exiting the water pump.
Once the silencer is mounted, you can trim the length of the expansion chamber tail until it all fits together. Again, trim little bits at a time until it is just right. It is much easier to trim a bit more off than to try filling in a gap with your welder.


I rotated the tail 90 deg before welding it on so that the threaded mount is pointing straight up when mounted on the bike. This should make it easier to make a mount to the frame.

At this point, the exhaust will be finished except for the mount that goes on the very bottom of the expansion chamber. This last mount will be added after the cradle is built.

Pipe Mod

I'll start by saying, modifying the pipe is a bigger headache than I anticipated. If you can buy a 125 to get all the parts cheap, then buy a 250 frame, I would strongly recommend it. But if you feel like you can't consider yourself a steely eyed missile man without doing it, then read on. ( I'm not a steely eyed missile man, I just cut my pipe up before realizing how big of a pain it would be.)

First make sure the engine is positioned where you want it. I put it as far forward as possible, which leaves the front of the cylinder about 1/4" away from the frame. About at this point the bottom of the engine hits the frame by the pegs, so you may need to grind down one of the tube welds a bit. This gives the most room for the carb, and allows the coolant ports in the head to fall into this nice little pocket in the tank. Surprisingly, tilting the engine forward makes the pipe interfere less with the front wheel. It is probably a good idea to have your head-stay mounts made before doing this, to make sure they will work with the engine position that you end up with. (I will post CAD drawings and images of the head stay mount shortly). But note that the head stay mounts will allow for a good bit of movement in the forward/backward tilt of the engine when they are not tightened down, so don't count on them to get the engine back in the same place every time. It is better to find a spacer to but between the front of the cylinder and the frame down-tube. Once the cradle is built, it will locate the motor. Generally the head stay is put on last after the other engine mounting bolts are tightened because it's purpose is to cut vibration and add rigidity to the frame rather than position the head.

Here is a good image of the problem. When the wheel is turned it interferes even more with the expansion chamber.
Start by removing the front mount. Remember to cut near the weld, not through the weld so that you can get through without cutting into the chamber. Be very careful to only grind most of the way through the tabs. You can then break the rest of the way through the weld by prying softly with a screwdriver. It takes patience do get it right. Once the tab is off, carefully grind the welds off. Again, take your time so you don't grind into the chamber. A sanding wheel on an angle grinder is much better for this than a grinding wheel.
Next we cut a section out near the inlet. You want to cut parallel sections so the pipe moves straight back. You can see that the section that I cut is on an angle with the inlet, leaving a wedge on the inlet. This is unnecessary, I later cut most of the material away to get the pipe tucked in far enough. This step is a matter of cutting, trying it out, then cutting more. Slowly sneak up on it rather than cutting out too much.You can hold the pipe in place with the mounting springs to check after each cut, this way you don't have to keep tacking it together then breaking the tack to shorten it further. You will have to shorten the springs as you go, which can be done with a strong wire snipper and a vice-grip. When finished, you will want about 3/8 to 1/2" clearance between tire and pipe, to allow for tire differences and fork flex. Be sure to turn the wheel left and right each time you check.
Once the front is tucked in enough to miss the tire, we then have to cut out a pie-shaped section to allow the tail to fit under the kick-starter and line up with the silencer. I choose to cut all but a one inch strip of the weld (marked in blue) to hold it together as I trimmed it down, but in retrospect, I think it would have been better to cut it all the way apart to allow cleaning of the metal inside the pipe. It is a bear to TIG a seam like this when old oil and unburnt exhaust gunk is burning and trying to escape. So cut it completely in half, right through the middle of the weld. Then us a wire brush on your angle grinder to clean the inside back about an inch from the cut. Do the same on the outside, removing the nickle plating if your pipe has it. Get it very clean or it will make you say #%@^%*!#! when you are trying to weld it.
I started with a very small sliver. Again, sneak up on the correct amount of removed material with patience. It is much easier to remove more material than to try to put it back.
Grind at it until it fits without hitting the frame, kick-starter, reed, case or shock. You may need to cut off the tail end of the pipe if it gets in the way while you are trying to get it right. We will have to shorten it anyway.
This is what mine looked like before welding. Note that the radiators almost fit once the pipe is modified.
Be sure to tack the tube about every 1-2 inches around the seam before starting the weld to be sure you don't end up trying to fill a gap at the end. Weld it in short sections, allowing the pipe to cool a bit between each burn. Otherwise one side may get hotter than the other, expanding more and causing a length mismatch. This can put extreme stresses in the weld. If you want the weld to look good enough to use without grinding, you may want to have a professional welder to do it for you. I'm no professional welder, so my welds may need a bit of grinding. Bad welders make for good grinders. Next we will mod the sub-frame to hold the silencer then we can shorten the tail of the pipe to fit to the silencer.

First Reassembly

Its time now to put the bike back together to some extent so we can see what additional mods need to happen other than the cradle. I replaced the chain guides and dust seals and re-greased the swingarm bearings.
Note that the swingarm bearing races are different lengths. the short one goes on the chain side.

Line up the frame, swingarm and rear engine mount, and slide in the swingarm bolt. Sight down the swingarm to be sure that the chain guide lines up with the pinion sprocket to make sure you got all the swingarm bearings, spacers and washers in the correct places.
It's good to clean and re-grease all the linkage bearings while you have them apart as well. When you pull the center bearing race out the pin bearings may fall out so be careful. Get them all out and cleaned up, then hold them in place with bearing gease. Install the dust seals with bearing or assembly grease on the inside of them, but wipe off excess on the outside once the bearing and seal are all back together to prevent attrackting dirt later.
I put the shock on to allow me to test the alignment of the air filter boot to the carb and be sure it doesn't hit the spring.
I rebuild the front forks with new seals and bearings. It is smart to do replace the seals and oil before they start leaking, to keep the oil free of solids. The forks will degrade very quickly if some of the outer tube aluminum starts to wear off and form a slurry with the oil. It is much better to replace seals early than a bit too late.

I always use genuine Honda parts for bearings and seals. I worked for a company that designed and manufactured seals and learned that just because a bearing or seal fits in the spot does not mean it has the right characteristics. Believe me when I say that an aftermarket supplier is not going to do a better job of selecting bearing and seal materials and design properties than the OEM.
If your forks have some substantial wear, and it looks like the oil is dirty or metalic looking, you will want to remove the cartridges from the fork lowers to allow you to wash all the old oils and debris out do the lowers and cartridges with solvent. If you try doing it while they are assembled, you won't be ably to get all the solvent back out of the cartridge, and it will cause your new fork oil to have a different viscosity. This takes a special tool, which I didn't want to buy, so I got out my angle grinder with a cutting wheel and modified the end of my floor jack handle. It worket fairly well. Another alternative is to remove the cartridge retainer using a impact driver. YouTube has some great step-by-step videos showing fork rebuilds by Rocky Mountain ATV. (They also have top-end rebuild and bottom-end rebuild videos.)
When you are ready to reassemble the forks use pleanty of assembly grease and leave the springs and oil out of the forks. We will need to be able to tie the front wheel up as if the forks were fully compressed to make the necessary mods to the pipe.
This is only necessary on a 125 frame. Without moding the pipe, the front wheel will bind on the pipe and may cause an unsoliscited breaking when landing a tall jump. You can imagine how unplesant that could be. Some people will buy a CR125 and use all the parts except for the frame, then buy a CR250 frame off ebay. All the 125 parts fit on the 250 frame, and this allows you to avoid the pipe mod. I've got a TIG welder so I don't mind the mods, and I like a few other features of the 125 that will be apparent later in the project.

Cut It Up

I've read forum posts showing people using cut-and-reweld, machined parts, additional sections of tubing, etc. to lengthen and re-shape a cradle to make it work. This seems like a lot of work to avoid finding someone to bend you some new tubes. On a 125, the lower cradle requires so much modification that I decided that it would be much cleaner to cut it out completely and have tubes bent to replace them. I called around to fab shops in the phone book and found a shop that can bend and weld aluminum tubing, who I will use when it is time to make new cradle tubes.
I pulled off most the parts leaving the sub-frame and handle bars so that I could stand it upside down.I used a reciprocating saw to make the first frame cuts. many people use die grinders, but I was advised to use abrasive cutting methods as little as possible because they embed junk in the aluminum which can cause problems during welding. That being said, I don't think I'll be able to do all the cuts with my saws-all.
As you make cuts, be sure not to cut to close to the frame leaving some of the weld on the frame. This is to prevent cutting into the structural meat of the frame. The welds can be carefully ground off later without damaging the frame. First we cut off the coil mounts and save them. We will likely weld them back on later in a different spot.



Next we cut the bottom weld on the lower radiator mount, and bend the mount out away from the frame. I'm fairly certain that we will end up cutting off all four mounts to reposition the radiators, but I'm going to wait until I have everything else in place in case I can leave them where they are.


Next we cut along the welds just below the bottom of the Y-piece from the back toward the outside. Be careful to be sure the blade is cutting perpendicular on both sides of the square down-tube.

Now we can cut along the welds of the front of the Y-piece.
Now that the Y-piece is loose, we can cut the back of the cradle. Cut within a few inches of the forging where the tube inserts. Later we will cut the welds and remove the square tube in preparation for inserting the new tubes.
The cradle should now be loose and can be removed. Save the cradle because we may cut the engine mounts off of it to re-use. All our carefull cutting may not be necessary; depending on how we do the Y-piece, but I figure it is easier to cut more off than to put material back on later.
Now we can slip the engine in the frame and get ready to start positioning all the peripherals (radiators, tank, pipe, carb boot) before we decide exactly where the engine will be set.


I noticed that most forum posts about these conversions start with putting the engine in and building the cradel, then the rest of the post is about fighting the pipe, carb boot, fuel tank, and radiators to make them fit where the engine is located. The location of the rear swingarm mount is set in stone but the angle of the engine can be adjusted to make all these parts fit the best that they can. As I play with the angle of the engine, it is apparent that small changes in position will make a big difference in fitting all the periferals. I think it will be easier to get all these components in place (and necessary mods made) and make sure they mount up, then build the cradle as a later step.

Doners

I bought a 87' CR500 for $250 a couple months ago. It was in very bad shape as the price tag might suggest. Before I could even start it I had to do a good bit of work to even get it to start. Once I did get it running, I could never get it to run quite right. It had throttle response up to about 3/4 throttle, then it would flatten out like it was starved for air.

After riding my friend's 89' CR500 which had that mind bending acceleration that CR500 are made for, I determined to cough up the cash and buy a real 500. I found a 2001 for $2000 a couple hundred miles away and had my brother pick it up on the way home from a business trip. It runs like a dream and is an all around great bike, although the 87' has a much sexier shape if you ask me.

I decided to pull the engine from the 87' to use for sizing while I do the frame conversion so that I could keep riding the 01'. There are only a couple differences to keep in mind. Here is a good link to a page detailing the differences among the different engine years: http://cid-76dd2330685afe13.skydrive.live.com/browse.aspx/Honda%20CR500R%20Tech%20and%20Reference/Tech%20Edits%20as%20of%20july%209th

The differences that matters for using an 87' to size up for 01' swap is that the intake on the cylinder is 8mm shorter on the 01' and the swing arm bushings are not the same diameter.

I pulled the engine, cleaned it up in some solvent and knocket out the rear bushings.

I took the bushings and the swing-arm pivot from the 125 to my machinist at work who bored them out to fit on a lathe. They need to have about a .005-.010 larger diameter than the swing arm pivot. Having a good machinist is necessary for these types of projects. If you don't know one allready, get in the phone book and start looking around. I've found that the best (and least expensive) machinists usually have the least expensive looking ad in the yellow pages. Most shops these days have CNC capabilities, but most of what you will need can be done on a manual lathe and mill. These bushings are as hard as the nubs of hell so don't get any ideas about buying a big drill bit down at the local home depot. Once they are cut, they are much thinner, but this should be fine since they only act as a wear barrier between the swingarm pivot, and the soft cast aluminum case. The swing arm pivot does not pivot in them, but all the vibration of the engine would eat away at the aluminum case without having them.

After cutting the bushings, I tapped them back into the case. Now this engine is ready to go into the new frame (as soon as the frame is ready).