December 22, 200718 yr Can a 125 pro top end be put on a 280 lower end? Would a stroker plate and/ or cylinder-case spacer be needed? This would produce a true 200.
December 22, 200718 yr I don't know the answer to your question, but I did have a quick ride on a true 200cc Gas Gas a few years ago that had been made by RCM Trial Sport of Cornwall, England. It was a while ago, and it may have been the old pre-Pro motor, but it was a super sweet little bike. Robin at RCM would tell you more I'm sure if you called him on 01209 820896.
December 22, 200718 yr Don't know what cc you want to end up with but julian Wigg made a 225 combining parts and a bit of machining, and a special s3 barrel. Based on a 125 bottom end, 250 top I think. Lovely little bike it was too, think factory kev has it now.
December 23, 200718 yr Would be an interesting project to do, it is physically possible but would need someone with a lot of 2 stroke port timing understanding and a bit of experimentation to match up the 125 cylinder to a 280 bottom end. The 125 motor uses a 54.5mm stroke, the 250/280/300 use a 60mm stroke. Even with fitting a base spacer to the 125 cylinder to end up with the correct piston height at TDC, (assuming the gudgeon pin is the same diameter for both capacities) then the port timing at BDC is going to be approx 4.5mm lower than a standard 125 motor. which I would imagine would have some effect on the transfer port timing. Still, if you have the bits, and the pin diameter is the same, it's worth a try. Bye, PeterB.
December 23, 200718 yr Author I have a friend with a couple of 280s. He would like something easier to start and does not need the power of the 280. It sounds like quite a few people converted 125s to a bigger bore so I figured that there should be some used 125 top ends for sale. If the 280 rod uses a bigger wrist (gudgen?) pin, it could likely be spaced down with a collar. That would even help the ballance. Machining and spacers could likely get a good mechanical fit but the port timing would be in question. I have some basic expereince with that but would have to get parts to measure the existing timing and calculate what the longer stroke would do.
December 23, 200718 yr A mate of mine told me about a 280 raga that had been fitted with a 200 top end, he said it was one of the best gassers he'd ever ridden..
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