XR100/RM80 Conversion Pit Bike Build On A Budget – Part 2

If you’ve seen other XR100 conversion builds, you know that this isn’t just an easy bolt-in process. It takes many hours (especially if you haven’t done it before) mocking things up, welding, making sure everything fits, and fixing anything that doesn’t work on the way. There’s some skill required (welding is the most important), but I believe most anyone that is mechanically inclined and motivated can do this conversion. You must have patience though, as there are a lot of little things that take time…

If you haven’t read Part 1 on the XR100/RM Conversion Build, click the link before you continue reading here.

Part 2 is the tear-down and engine mock-up process. In order to get an XR100 engine in an 80/85cc mini motocross bike, you have to take everything off so you can cut and re-make the frame cradle. Getting the frame and engine mounts right is probably the most important part of this build. Not only does it all have to fit, but if you make the cradle wrong, the shift lever, rear brake pedal, or kick-starter may hit the frame. More on this later, as it is more difficult to explain without pictures. Here’s the tear-down:

’97 RM80

Here’s the bike stripped and ready for mock-up.

In order to get the xr100 engine in a mini motocross bike frame, you want to place it as far down and back as possible. And to do that you have to cut off the lower mount on the engine cases. Below is a picture of what it should look like. You can cut it off, mill it off, grind it off (just remember that it’s aluminum), or any other way you can get rid of it. This is so you can set it back in the frame. In addition, it lowers the center of gravity on the bike (better handling), and there’s more room for the cross-brace above the cylinder head.

XR100 bottom engine mount cut off

Now it’s time to start making the engine mounts. Luckily I have a junk XR100 engine that I can use to make my mounts since I didn’t have an XR100 dirt bike at the time. Here’s the material for the rear engine mount:

Rear Engine Mount Materials

I used 3/4″ round stock and drilled it to the size of the bolt (didn’t have any tubing at the time). The steel plate is just some 3/8″ that we had laying around. I tried my best to center the pieces so everything bolts up straight. Other than the tools to take the bike apart, the next tool you will need is a welder. This is probably the most expensive part of your project, unless you have someone doing the welding for you. However, I highly recommend getting a just a MIG (wire-feed) welder if you plan on building more projects or repairing stuff yourself. I use a Lincoln 180 MIG welder because it’s simple, reliable, and relatively cheap. They are very useful and can save you a lot of time in the future. A MIG welder is fairly easy to use, but I suggest getting the basics taught to you by a local welder/friend. The goal on these engine mounts (and pretty much any welded part) is to get the best penetration so the weld is nice and strong. Remember to always clean the metal before welding, otherwise it will turn out looking like pooh and the weld will be more susceptible to breaking.

Rear Engine Mount Welded

Below is the tube pieces welded to the plate and mocked up on the swing-arm bolt. The smaller length tube (lower) has a dimension that allows it to just barely fit in between the swing-arm. The longer tube length is suppose to match the length of the bolt tube on the xr100 cases. That way I can make tabs on it with holes for the bolt to go through. You’ll see in the next picture.

As you can see, part of the mount on the right case broke off. Fortunately for me it didn’t make much difference because I could still get the full length out of it. You want to take your time with this mount, making sure everything is straight, lined up, and the engine is where you want it to be.

All welded up with the swing-arm.

Below is an estimate of what it should look like. My research found that you should have the counter-shaft sprocket aligned with the swing-arm bolt; both up and down, and side to side. This is so the chain is straight on the sprockets and rollers and won’t fall off.

Centerlines of the Sprocket and Swing-arm aligned

We’ll go over the front engine mount in the next article because that is when I’ll be making the frame cradle. So stay tuned, because this will be the biggest part in the conversion! Here’s the link for the next article: RX100 PART 3!

-Tom Stark