Soon this will enter my dyno. I am almost done with the porting, some minor stuff still to do. Be sure to follow me to see the result from this. We reached 22.1rwhp with unported cylinder. 50mm bore, 45mm stroke.
You are talking about the cylinderskirt? I actually don´t know the dimensions of a oem yz cylinder so i can´t answer you. But! As it is modded to sit on a Zundapp engine it might be a clue. But also, i have checked, the piston is contained inside the sleeve during whole stroke. So there are no issues =)
