# 1973 Pontiac 350 with 2 different cylinder heads



## Cafr119 (Dec 9, 2017)

Hello Forum, I’m looking for the Pontiac Motorheads, ok, I have a 1973 Pontiac 350 that someone at some point has installed 2 different Cylinder heads, on the left side is a 4X head and the right side cylinder head is a 6X. For the life of me don’t understand what would the purpose of this ??? And what does this do to the running of this motor ??? Now I do know and love GTOs and know a bit when it comes to the Pontiac engines but I’m not 100% on valve sizes and what heads have what and I have researched the Wallace Racing write ups but , by putting 2 different cylinder heads on a block is this good ?? Thank you


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## LATECH (Jul 18, 2018)

Pontiac V8 Cylinder Heads

Click the link, check it out


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## Cafr119 (Dec 9, 2017)

Ok I read the link, as I mentioned I have read this before, so, I’m looking at one head 6X could be a 75-77 head with bigger valves than the 73 4X head, bottom line I find either a 4X or a 6X to make a matching set


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## LATECH (Jul 18, 2018)

With a 350 you probably wont notice much of a performance issue. They run good but arent a powerhouse. 

If it is for numbers matching /GTO engine like the 73-74?


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## LATECH (Jul 18, 2018)

The 350 block (72-74 IIRC) is chamfered at the cylinder tops to give extra clearance for valves as they have a small bore and the flow is somewhat shrouded on the cylinder side because of the proximity of the valve.

Bigger is better, but you need bigger valves AND cylinders...LOL


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## Cafr119 (Dec 9, 2017)

The motor came out of a non number matching 73 GTO. I was going to use it in a project car but, I’m going to buy either a 4X or a 6X to match one or the other, it doesn’t matter to me and as far as power, just building a nice lil’ cruiser, but I appreciate the info, and I never had any issues with any 350, Chevy or Pontiac, I’m happy with a good running motor any day.


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## Cafr119 (Dec 9, 2017)

Well, as I’m doing more research on this motor, the 4X head has a letter “H” stamped on the small flat spot on the front of head, checking the date, working on the 6X head now, block I misspoke myself is a 1972 350


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## LATECH (Jul 18, 2018)

Cafr119 said:


> The motor came out of a non number matching 73 GTO. I was going to use it in a project car but, I’m going to buy either a 4X or a 6X to match one or the other, it doesn’t matter to me and as far as power, just building a nice lil’ cruiser, but I appreciate the info, and I never had any issues with any 350, Chevy or Pontiac, I’m happy with a good running motor any day.


Yeah, Cool. Its not about what anyone else wants ,its what you want! If it runs good and looks cool and you like it, that is what matters most.
Best of luck with you project.


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## Cafr119 (Dec 9, 2017)

?✌


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## Cafr119 (Dec 9, 2017)

Ok, it looks as if the block and the 4X are 1972, the right head must had a issue and someone put the 6X head, now when I bought the car from the kid that sold it to me, told me the previous owner told him this 350 has a mild cam, but now I’m thinking it has to do with mismatched cylinder heads


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## LATECH (Jul 18, 2018)

If you are thinking it run roughs or is lumpy because of the cylinder head mismatch , then perform a compression test on all 8 .
If the one bank is more than the other, and consistent , then you know the head is messing with the CR.

Might be wise to check the engine either way , just to see what you have . If you have a larger chamber 4x and the smaller 6x they may be pretty close to equal on the CR

If you have one or two low cylinders out of the bunch, then you may have a valve or ring problem.

Do a dry Compression test first, then do a "wet" test. Keep track of results (Make a chart write it down)

Keep us posted


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## Cafr119 (Dec 9, 2017)

Well , I’m going to tear down the motor, I’m going to part this motor out, I’m going to sell the Edelbrock intake, the Edelbrock carb which I think is a 1405 or a 1406, I’ll have to look up my notes, I have another motor that I’m going to use on my project, this motor has too may red flags for me.


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