what is a good and safe bore 30 over ,,,,,,,125over what?

0.060" is very safe. 0.100" over is quite common with racers, although some feel the need to sonic check. 0.140" is about the max safe overbore I've heard - good idea to sonic check.

Lou

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Home of Slant6-powered fun machines since 1988

It depends on what you want to do. On a stock rebuild, +.030 would be fine. For any kind of performance build, the bigger the bore, the better the head will flow. Mine has always been +.065 on my street motor, and that was mandated by the pistons I used. My next rendition will be +.100". +.100" gets you into a bore size that is somewhat easier to get pistons made for.

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Official Cookie and Mater Tormentor.

How about 170. Is it safe to bore to 89mm, ~+.100? It´s going to be turbocharged and will see lots of boost. Or should I stay with 88mm bore, ~+.065. The same piston is available in 87mm, 88mm and 89mm.