Sunday, February 26, 2012

1979 Puch Maxi CRF50 Engine Swap

This Maxi is actually a pretty nice example. I rode it around,
got bored, and sold it.

Here's a 72cc pit bike I picked up at a yard sale for 250 bucks.
Looks like the dude also had some sweet mud tires up for grabs.
You can find the entire CRF cheap used, or buy yourself just the complete engine from Ebay. If you just get the engine, make sure you pick up a throttle, cables, and a kill switch.

Some special tools you'll need:

Welder - Wire feed seems to work okay for me. In Don's Garage, I don't use gas with my welder, just flux core wire. A cheap ass stick welder will work also, but may be sloppier.

Angle Grinder - Make sure you have cutoff wheels, a grinding wheel, and a wire wheel is really helpful.

Shop Tools - Hammer, wrenches, pliers, sockets, drill with good bits, and maybe a Sawzall.

Commitment - You're potentially going to ruin your Maxi if you mess up.

Some supplies to get your hands on:
  • Chain - I don't remember what size chain it will take, but I do remember that it's the same pitch for the CRF front sprocket, just with wider links for the Puch rear sprocket. You should be able to find it at a place like Northern Tool.
  • Commitment - Don't plan on doing this on a Sunday afternoon, and its your only way to work on Monday morning.
Get Started

First up, you'll need to dismantle the Puch. This means draining the fuel and removing that piece of shit engine from the frame. You'll also have to remove the pedal assembly. This just takes a couple of hours to do if you're a normal person, and about 15 minutes if you're a Spartan.

I notched the shit out of the frame to make this 72cc motor fit.
Just take your time. You can always cut more, but
un-cutting (welding) is sort of a PITA.
Frame Cutting

Clearance the Puch frame around where the CRF engine will mount. You'll have to make room for the upper part of the motor. Use your angle grinder for this. Please wear safety glasses. I usually rock safety glasses, work gloves, and hearing protection.

In this picture you can see an engine mount peeking out from
behind the exhaust.
Engine Mounting

Fab up some mounts for the new motor. I used the mounts that were welded to the CRF frame. Just cut them off, bolt them to the motor, put the motor in place, and weld to the Maxi frame. Make sure you check your chain line (twice). This is really critical. My moped goes about 55 MPH, and throwing a chain at that speed sucks and is dangerous.


Get your chain and make it the right length. You don't need a fancy go-to-meeting chain tool. Grab your grinder and grind off the rivet heads, punch them through, and you can take the chain apart. Use a master link to reinstall.

Fuel System

All I did was run a fuel hose from the stock petcock to the carb. At this point, your frame should be empty... Just fill it with regular 87 octane. Honda engines don't care. The first couple of tanks of fuel may burn smoky because there will be some 2 stroke oil residue left in the frame.


The CRF50 throttle cable should reach. You might have to extend the kill switch wires. You can probably also find a longer cable that's meant for the CRF engine.

The Maxi at Alterra. That's where you go to let people
watch you type on your laptop.


  1. does it have a transmission now?

  2. This comment has been removed by a blog administrator.

  3. So haw you fucked around with a 88cc or bigger 4 speed? I always wanted to install a 125cc clone 4 speed into a Magnum MKII maybe even go manual clutch.

    1. I'm pretty sure that all the clone engines are pretty close to the same size externally so they should work. The closest clearance issue was the front tire to the cylinder head. I wasn't able to run a front fender, but you could probably just cut it short so it doesn't hit. You could also notch the frame farther back to set the engine in a better location.

      125cc with a clutch would be awesome - there's no reason it wouldn't work.