how far can you bore stock cylnder??

[400ex18]

???? is it 68mm or can u take it bigger?

[Qracer]

I know that the 85-86 cylinder can go to 69.5mm. As for the 87-89, pistons are available up to 68mm.

[Dave83]

You can go bigger than 68mm with a 87-89 cylinder,but you have to use the 85-86 style piston and a spacer plate depending on what length rod you are using.Wiseco was offering pistons up to 69.5mm I think.

[400ex18]

ok thanks guys!!

[C-LEIGH RACING]

When you get done with the 69.50mm piston, dont throw out the cylinder just yet, because you can bore in a 70mm 86 model YZ 250 piston. You'll need a 2.5mm spacer plate though.
Neil