400ex overbore!?!?!

[zoom400ex]

I was just wondering because i saw it in the april dirtwheels mag that a 400ex can be bored for 0.10 to 0.40 over and still use a standard bore piston...........is this true?.... will it effect performance if i use a standard bore piston with this size overbore?!?! any input would help. thanks

[mikeboone]

Dont even try it. Any piston from .10 over to .160 over MUST have the cylinder and sleeve bored correctly. The only purpose in boring the cylinder is to install a bigger piston for better performance.

[zoom400ex]

thanks thast all i was looking for..................guess i'll go with the 416 then........

[mikeboone]

Ok, here is what i'm saying. The 416 is an 87mm piston. This is .080 overbore and will require the cylinder bored to specs. Not some # pulled out of a hat but THE specs stated with your new piston. This will only work with that piston. A 426 (88mm) or 440 (89mm) piston will NOT fit and vice-versa. If you did bore the cylinder .10 over and ran the stock 85mm piston; oil would explode by the rings since the 'hole' is too big. I cannot beleive the Mag said you could run a bigger bore with stock piston. :rolleyes:

[mikeboone]

Zoom; perhaps you read that you can run a stock bore HIGH COMPRESSION piston? That IS possible w/o boring anything. Stay below 11:1 to run pump gas. It will have a good bit more bottom end with this piston.