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I'm in the process of rebuilding a 389 out of a 1964 GTO. I'm using the original heads (9770716) and needed to drop the compression. I found some SpeedPro cast pistons (+.030") and was going to machine a dish in the tops, but my machinist said a simpler option would be to use a thicker Cometic head gasket, so the pistons were not dished. All of the machining on the block is done, everything is balanced and the short block is assembled. I called Cometic for the head gaskets and they do not make gaskets for a 389, but they do sell head gaskets for 1968 and up 400's. Cometic said these gaskets do not work on a 389. Does anyone know if a Cometic gasket for a 400 can be modified to fit a 389? Does anyone one know what the difference is? I really don't want to rebalance everything again, but it looks like that is maybe what I'll have to do. I've heard stacking two head gaskets will work, but that makes me nervous...

Thanks. Any advice is appreciated.
 

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Summit says the Fel-Pro head gaskets with the thickness of .039 is the same for a 389 and 400 are the same.

they also sell a copper one that will get you .062

I would not stack 2 together. I did it on a Camaro and had lots of sealing issues.
 

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welcome! the route of obtaining and installing the thick Cometic head gaskets, not only expensive, but the big space is going to kill quench. A bad thing about cast rebuilder pistons, most juggled the pin height a little to lower static CR, and top of the pistons will set a little further in the hole, again killing quench. While the result has lowered static CR, much needed quench is gone.

Haven't examined a set of cast rebuilder pistons for 389 in ages. it would concern me on a set of cast pistons that good chance the deck thickness on the piston is too thin to machine a propper dish. The last cast set of rebuilder pistons I had in a running Pontiac was bought in a "local yokel shop" built '67 WT 400 which came in a package deal with my buying several '67 and '68 GTO projects. Traded that '67 WT off, as it didnt belong the either of the '67's, and a car builder wanted it for codes and dates. Long story short, the builder finished and sold the car, new owner has it in the 67 GTO, trys to run it on 91 octane, and within a year, detonation killed the top of several ring lands. I ended up being referred the new owner, and tore down and sent the shortblock out, had it built with new forged dished pistons and had the recip asm balanced. End use customer, very happy.

In your case, today, I would look at a set of affordable custom forged pistons from one source or another. A set of AutoTecs, properly dished, ought to run around $450 plus the cost of machining the dish...most likely another $150-175.
 

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Discussion Starter #5
Thanks to all of you for the advice. I'll check into the AutoTecs forged pistons - that sounds like the route to go.
 

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What Pinionhead said, 100%. You need the right pistons, not a patch job with thick gaskets which will result in lousy quench, detonation, and a poor running, short-lived engine. You first option of machining the heavy forged pistons would have worked just fine. Custom pistons are better, though. Good luck, and thanks for posting.....I'm thinking you dodged a bullet here!
 
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