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Discussion Starter #1
I know this has been discussed before but I was asked by a forum member to present my specific question. Will 6X-8 heads fit my 389 as currently built. My engine has the 8 valve relief pistons with the knotch at the top of the cylinder wall.The pistons appear to be at zero deck. I could buy a 400 head gasket to see if everything on my 389 lines up but would like to hear from those of you who have hands on experience with this type of set up. I havent had the cam grind checked but it doesnt appear to have a high performance lift. Will this work?
 

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No hands on experience. But it should work.

Will have low compression. Will need a '72-'79 or aftermarket intake. There ARE ways that will allow you to use earlier intakes with the 6X heads. Some have said they cut out a thin piece of stainless steel, to seal off the area around the passenger side exhaust heat crossover hole. Since the 6X heads have a blind hole above the heat riser hole in the head, that causes a leak when using the '71 & older intakes, which have a large rectangular crossover hole on the passenger side.

This article explains the problem better than I can.

https://www.hotrod.com/articles/hppp-1301-pontiac-cylinder-head-modification/

Some have posted that using Fel-Pro 1233 intake gaskets will work. I don't know if that's true or not. But I have read where guys have used a homemade thin stainless plate that worked. Whatever method is used, that area must be permanently sealed off, to prevent an exhaust gas leak.
 

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Discussion Starter #3
Thanks bigD. I have been trying to decide whether to put the 6X heads on to lower the cr or have my "76" heads rebuilt with threaded rocker studs and have the valve train set up correctly and hope it will run well on pump gas.
 

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You shouldn't have an issue. Those 8-relief pistons are not optimal, though. I've run 400 pistons in a 389 with no issues running the 389/421 heads. The trick is to not have a cam over .500" lift. If I were doing what you are doing, I'd search for an 87-90cc head for at least 9:1 CR. I installed a set of 87cc #15 455 heads on my 400, and they work very well. I can now run 91 octane pump gas, which would not work with my old 670 and #12 heads.
 

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Discussion Starter #5
I would be very happy to get a 9.1cr and run 91 pump gas . I also would like to thank you all for sharing some ideas and options that will give me a dependable Pontiac build.
 

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# 11 heads ( 1970 350) have 85 - 87 CC chambers. That would put you a tad over 9 to 1.

I just aquired a set, and I am not interested in selling them, but I wanted to confirm the chamber size.I CCed them and I cam up with 85 CC. They were never off of the engine I tore down.
Might be an Idea to look around for a set for your build.

with 8 valve relief pistons, they will clear just fine. Remember that early heads had a different valve angle ( 22 Degree valve angle IIRC) and pushrod length is different than a poncho motor with later model heads.(14 degree valve angle IIRC)

Just something to be aware of
 
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