250r bore size?

[440exwlspankyou]

I have a 66.5 mm bore on an 86 250r cylinder. What is the piston that I need? Like .20 over or .30 over I am a 400ex man and I am clueless about 250r's. I am doing a go-kart project for my boy. thank you

[exboy400]

you would need a .50 over piston...making it a true 250cc

[440exwlspankyou]

Thanks that is a big help. I am doing a retrofit. I am putting an 86 cylinder on an 83 block. So far my mods are:
1. milling the top of the cylinder
2. ovaling the base bolt holes
3. custom aluminum base gasket
4. external water pump
5. drill/tap head bolts holes

When its done it should be a smaller bore and smaller stroke It should be a 220 when I am done ie. 86 bore w/83 stroke. I will get a high comp piston to help restore power.

It started as a easy fix that turned into a major undertaking. The only reason that the project continues is because people said that it couldn't be done. Project: "250r headache" will continue!:devil: