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Forged Pistons

562 Views 1 Reply 2 Participants Last post by  jon3k
Hey everyone- i have a questions that is probably pretty simple for most of u to answer. I am getting ready to put forged pistons into my 91 Stealth Turbo. My question is: i dont know how much i can safely over-bore the block. The pistons i have been looking at are around .039"-.040". Is that to much to bore our blocks safely? Does anyone know what the limit actually is- any help would be great-thanks.
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"A question often asked when rebuilding an engine is "How much can I overbore the cylinders?". From the service manual engine cross sections I guessed that the cylinder wall thickness might be 0.25". Joe Gonsowski (3SI Message Board) had his block sonic tested (top, middle, and bottom of bore) during his rebuild. Average wall thickness was 0.288" with a minimum value of 0.231. The stock bore is 91.1 mm (3.5866"). I had my engine bored 0.050" over stock (92.38 mm bore for a displacement increase of 84 cc) which reduced average wall thickness to about 0.235". After only a few thousand miles I have not yet encountered any problems, but the increase in wall deflection and stress may reduce the durability of the engine somewhat. Some of our cast-iron block 6G72 engines have been bored 0.075" over stock (a 93 mm bore, increasing displacement by 128 cc) bringing total displacement to 3100 cc and average wall thickness to near 0.200". I have not heard of 6G72 engines with larger bores than 93 mm. "

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