I have a weisco piston (86.5) in my zilla. I hear everyone say there's is 20 over or 50 over or 100 over ect... What would mine be and why? I guess it's important to know these things about your machine, IDK. Appreciate any input. Thanks

When people say 20 over, them mean the pistons is .020" bigger than stock. .508mm = .020 inch.

These are overbores.
.25mm = .010"
.50mm = .020"
.75mm = .030"
1mm = .040"
ect

If stock piston size is 64mm, and you have 64.5mm, you have a .020" overbore piston in there.