YZ 100 Rebuild: Bore Size

[Joe]

I'm currently rebuiling a 1976 YZ 100 and have already determined that I'll need a new ring and probably a piston.  I'm just having trouble figuring out what size I need.  I measured the bore of the cylinder- 51mm and even though I measured several different spots and got the same measurement it appears to be egg shaped.  Even if it isn't egg shaped I haven't been able to find the cylinder to piston clearences on the internet and so haven't been able to determine the size of piston and rings I'll need.  If it isn't egg shaped what piston size will I need?  Anybody got a chart or just know what bore is needed for each or any size of over bore pistons?(.5mm over to 2mm over)  Is there 0 clearence between piston and cylindar?  Thanks  

[eprovenzano]

I'm not familiar with that bike, but is this a pre-power valve bike?  You may have to call a piston MFG'er and ask for their opinion.

[SachsGS]

It sounds like you have a badly worn cylinder that will require reboring.Take your cylinder to a machinist and have him (or her) measure the bore of the cylinder at several different places (typically you would use an inside micrometer) to determine maximum out of roundness.From these measurements you will know what the cylinder will have to be bored to in order for it to clean up.This will give you your piston size.The piston manufacturer will supply the recommended clearances.

P.S. While you have the cylinder off you should also check the con rod.A quick and dirty way to check the rod for play is to pull up on the piston with one hand and tap the top of the piston with the open palm of your other hand.If you hear a "tink" sound you've got wear (and trouble) there as well.