Vintage Dirt Bike Q & A

I plan to put a 76 MR175 engine in my 75 CR125. Will the 75 CR125 crankshaft fit into the 76 MR175 engine? I want to be able to bolt on the CR's ignition rotor so I can keep the CR's CDI ignition. The crank "wheels" ("cheeks") are a different shape. Maybe they need to have a different balance to match the greater weight of the MR175 piston so the engine doesn't vibrate. Has anyone done this? Advice appreciated, thanks.
Mark

The CR125 crank has different size rod bearings on both ends so it will not go in the MR175 motor. I think you can space the ignition on the MR175 crank so you can use the CR125 type ignition. I want to try the opposite mod by using two MR175 crank halfs with a 1981 CR125 rod kit which should mount to the MR175 crank without mods. Don't know how much vibration the MR175 crank will cause though.

Hi! I have done both mods, and it is not to complicated at all. The CR125 CDI ignition will work just fine on the MR175 crank, (stroke is the same as a 125) and will give you a little quicker throttle response. If you are going to put the MR175 carnk into a CR125 bottom end, you need to machine the cases ot accomodate the slightly larger crank weights. It is not to tough do this, it is only a millimeter or two. They all use the same main bearings, so no worries there. If want a real screamer with a bit more power, the MR175/CR125 transmission is a great setup. Good luck!

Thanks. The CR125 rotor and stator bolted right on. Easy!!
The CR125 ignition cover fit on too.