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Unread 11-26-2001, 10:08 PM   #1
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Join Date: Nov 2001
Location: Edmonton AB
Posts: 97
Oversize Pistons

I own a 1987 YJ 258 I6 with a 4.0L head swap. I'm sending the bare block down south next week to get cleaned and magnafluxed. Is it worth my while to overbore the cylinders to gain HP? If so, how much con I overbore the cylinders by and what size pistons will I need?

89' YJ
AMC 258, all mods
7" SOA BF 35" mud terrain
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Unread 11-27-2001, 12:19 AM   #2
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Location: KS
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Your best bet is to check with the machine shop doing the work to see what they think they can bore it out to. You better off just cleaning the cilinder wall and going with a little higher compression pistons to make the horse power. A good 9.5:1 or even 10:1 is nice but you will have to use premiem fule. But than again the price you pay for power.
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Unread 11-28-2001, 08:11 PM   #3
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Location: Maine
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Even if you went with the biggest overbore pistons available 60 over or .062. Your 258 would only end up being a 267.
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