GPR Camp: Replica Racers

I have a 2000 GPR 80 (EBE075). The rear sprocket is in need of replacement. As I look into acquiring a sprocket, there seems to be a model year break at 2004. After 2004, rear sprockets are fairly available. I can't find rear sprockets that purport to fit a 2000 GPR.

Are the rear sprockets really different? Sometimes the parts are the same, but the listings are inaccurate.

My center hole is 108mm, the bolt circle is 123mm, the chain size is 420. My 2000 appears to have a built-in offset or dish. Perhaps that is the difference? Are the later sprockets flat instead of dished?

Any suggestions?

There are a number of ways to go. Basically you wont easily find the stock sprocket, which is something around a 46 tooth. I actually can't remember the number at the moment. If you could count yours that would be nice.

What I did with my EBE075 bikes is install the 52 tooth sprocket on them that's employed on the 50s. I don't really suggest this though. It mostly happened because I just swapped 17" wheels on the bike, and they had a 52, or I put an EBE075 into a 2004 bike.

If you decide to go that way you can order either a 52, or 53 from one of the 50cc GPRs. Another option is to get as custom one made. Sprocket Specialists can make one for you. I have a few of their sprockets that came with one of the track bikes I've bought.

Another option is to source a sprocket from a Sachs MadAss because the 50 and 125cc bikes employed the same 16" Grimeca wheels as the older GPRs. These are available as a 46 tooth, which just might be the same size as fitted on your bike—although I'm not sure about the dish factor.

Pretty sure my 2000 GPR EBE 075 has a stock 42T rear sprocket.

I went up 1T on the countershaft sprocket to drop a few revs, and it's been fine. My bike has a 24mm Flatslide and an Arrow pipe, and once jetted properly can pull the taller gearing without a problem.

There is an offset to it, so a flat sprocket would not line-up.

Ah..... there it is again, those bloody 16" wheels. They run a weird sprocket! Until I find a set of 17" wheels, I'll just have to deal.

I have a thought on adjusting for the dish. The dish is +- 3.5mm deep. How about using a flat sprocket, but shimming the counter sprocket inward 3.5mm with a 3.5mm shim just under the sprocket lock. There may just be enough room on the output shaft. I'll try that.

Again, thanks for your input on my racer project.

From my understanding the 16" and 17" three spoke wheels from 2004 back are the same in the hub area. So there's no difference from the wheels being 16" in this case. Wheels from 2005 on are different, but they wouldn't fit on your bike as they are wider.