Vintage Dirt Bike Q & A

Hi,
I've been lurking here quietly for a while, usually finding the info I need from old questions asked, but I'm not sure about this one. I've got a 75 CR125 with an MR175 engine installed, and I have to split the center cases to replace the kick starter return spring, and I was thinking I should try to improve it a bit while I had it apart.

I've read here that you can replace the MR point ignition with a CR125 system, but the 125 has a bolt and the 175 has a nut on the crank end. What kind of puller would remove this setup on the MR? or is there a 125 rotor that is designed for the MR crank end (76, 77 maybe)?

I've also read that you can replace the transmission with the CR, does anyone know if you need to use the clutch and crank gears or the sprockets from the 125 as well to make this work right?

Thx
Hans

Someone else will have to help you with the transmission question but I can help you with the ignition.
To hold the flywheel from turning while I loosen the nut, I made up a bar with two bolts through it spaced apart to fit into the flywheel slots. A hole in the bar between the bolts lets my socket wrench reach the nut on the crank.
I use a 2 jaw battery post puller to get the MR flywheel off. Tighten it up and whack on the end with a hammer. Be careful that you don't damage the coils behind the flywheel with the jaws of the puller.
Remove the MR stator.
The CR rotor will fit onto the MR crank. Re-use the MR nut and washer. Install the CR stator and align the marks.
A CR ignition cover will fit over this CR ignition setup so you don't need to use the dorky looking MR ignition cover.

Thanks for the info, I was concerned about getting the 125 rotor off the 175 crank end. With the crank protruding from the rotor, there's not much room left for a puller to get at the internal threads on the rotor. I was hoping I wouldn't have to use a jaw puller on it, I think the back is aluminum.


Thx
Hans

To get the CR rotor off an MR crank I just remove the stator then use a 3 jaw puller and haven't had a problem. I've never used the rotor's internal threads for pulling.

Again thanks for the advice, I looked through the bottom ends I have laying around, and one of them is a CR125-M3... This one has a crank like the MR175 and a rotor designed to have a nut on the end. I had actually thought all the 74-77 were the same.

Thx
Hans

I would use the MR175 tranny in the MR175 motor. The ignition swap is a good idea but the MR175 has more torque and the six speed 125 tranny will be a pain unless you want to shift very fast. I tried this setup and put the MR tranny back in the cases. You don't need the close ratios of the 125 tranny.