QuestionI'm putting a cb500 big bore sleeve into a 125 with a cb750 piston, old time stuff says single valve pistons K models 61mm, what about a two valve piston, it seems to have a higher dome?
AnswerMike... gotta clarify if you are speaking about SOHC vs DOHC pistons or something else??
The combustion shape of using a 4 valve piston in a 2 valve combustion chamber would probably upset the flame travel.
With that much more bore, even a flat top piston will be giving you some big compression jumps, if you don't relieve the combustion chamber bore a little bit more.
Check with www.powroll.com to see what they recommend for big bore 125 motors. They have been building that stuff for a long time.
If you have the one-piece head engine, keep a close eye on the camshaft fit in the head. They do get loose/worn and then the ignition timing starts wandering around as the loose cam changes the point gap. Disregard if you have 2 piece head and/or CDI ignition.
Bill Silver
