# 389 Piston size



## y2kjammer (Dec 18, 2009)

OK. The engine is at my builders. The 389 block had original pistons stuck in the bore. I'm going through the block, but need at least .30 overs. Question, do I go .30, .40 or .60? actually .40 is a mute point. it's either .30 or .60. Any pros or cons? I can get new pistons at the local speed shop in any of the sizes.
Thanks

BTW: I'll be running an origina; 1966 Carter AFB (625 cfm?) and original cast intake and original cast exhaust manifolds and a cam spec'd pretty close to an 067 or 068. I'm trying to keep it as stock looking under the hood as possible (389 LOOKS like a 326 from the outside...)


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## geeteeohguy (Feb 2, 2008)

Go as little as possible. .060 is the max safe overbore on a 389 due to cyl wall thickness. If you go .030, you can rebuild it again later at .040 and still later at .060. Boring 60 over from the get- go is cutting the lifespan of your block short. It's like taking a new disc brake rotor and cutting a bunch of material off of it and putting it on the car. If you go .060, you will not be able to bore it again, you'll have to sleeve the block to be safe. I've heard of some going .090 on a 389, but that scares me. You can get custom pistons made in .005 oversize inclements. I would bore it as little as possible for clean up. It will probably need .030 if never done before. Also, if using your stock cyl heads, you will need to use some dished pistons from Ross or Pacific or elsewhere to be able to drive it on today's gas.


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## y2kjammer (Dec 18, 2009)

Thanks for confirming what I was thinking. I don't want to max the block at this point if I don't need to. .30 over it is unless there is a need to go .40, but I doubt it. My builder will be working the pistons and doing a bit of cleaning up on the heads to get me down to ~9.5:1. Appreciate the feedback.


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