Gearing for a 125 in a 70?

[pismorat]

I have an '82 ATC70 and just bought an '01 XR70 with a clutched 125cc engine in it. The engine has no counter sprocket, but the rear is still a stock XR70. What tooth should I put on the front to compensate for the larger motor? I someday plan to put this in the ATC70 when I prove it to be a good motor.

[pismorat]

If it helps, the stock is 15T front and 36T rear...

[Thorpe]

Rock that, and see how it runs... Alot of those chinese motors have different sprockets.... (stock honda ones dont fit)

[pismorat]

If that is the case for this one, where do I get the sprocket to fit a Chinese motor? This one says CDI on the head cover.

[Race Demon]

13 or 14 with stock tires is a good start with a 125, I run 14 and 15 tooth on my race 70s, From mad 70s

[Race Demon]

go to crf50 on ebay and check out the sprockets, if you look you can find some of the sellers that will tell you 17mm or 20mm shaft size, most of the time they are the same as Honda but like they say there are some odd ones Buy the way, 420 and 428 chain size will screw you up to stock 70s are 420

[pismorat]

Thanks, I'll measure the shaft (in between the cogs I assume) and see what it is, then make sure I check it out before I buy one. Thanks also for the chain info, I never thought I would put a bigger motor in my ATC70, but since this one fell in my lap I am kinda juiced about it. It's nice to know the little tricks up front.

[pismorat]

This motor's counter shaft measures about 20mm to the OD of the teeth. I am going to order a 15T sprocket with a retaining plate and bolts.