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Old 12-25-2012, 11:07 AM   #1 (permalink)
 
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Piston replace possibilities

After buying a badly running 1967 with a 1969 xh engine (350 hp)(48 heads). I find that it has at least two broken pistons. The car is an old drag racer and was probably over revved.
I am not inclined to drag race but would like a nice driver- is it possible to lower the compression (now 10.5 1) to say like 9.0 or 9.5 to 1 to make pump gas more acceptable or is this necessary? Since I am having to replace the pistons anyway, this would be a good time to do it. Are you guys running these HO engines able to get by on premium pump gas?
Oreilly,s only show 7.6 cr pistons or 10.75 cr. Is there a source for custom pistons with different compression ratios.
Also, I plan on driving this maybe 1000-2000 miles per year. Is it necessary to harden the valve seats while the heads are off?
I am impressed with the forum- please advise
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Old 12-25-2012, 12:12 PM   #2 (permalink)
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With the engines history I would recommend you check the block and heads for additional cracks before sinking alot of money into a rebuild. If the valve seats were not hardened prior to the high performance build you may have cracks in the valve seats.

Just my humble opinion and welcome to the forum,
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Old 12-25-2012, 02:14 PM   #3 (permalink)
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Where to begin...this has been covered a lot here and on the other forums. You can do a search.Very briefly: no, you can't get by on pump gas. Cast pistons break due to pinging caused by pump gas. I have done this. O'reilly is not the place you want to get anything for your rebuild. You can : get custom dished pistons and keep your heads, or get forged flat tops and change to bigger chamber heads. You will need to change the rods and get forged ones. Do some research here and elsewhere and learn all you can. Ditto on checking the block and heads out really closely since there were parts failures.
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Old 12-25-2012, 03:51 PM   #4 (permalink)
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Yepper, there's a lot to the topic. Realize that although the factory rated them at 10.75:1 they weren't really that high in practice. That figure was based on a 'factory blueprint' combustion chamber cc size in the mid 60's and a block that had been zero-decked. Actual chamber size on "most" Pontiac iron heads of the era will be around 72 and pistons are usually around .020 down at TDC. To find out where you're really at, compression wise, you need to actually measure both your chamber sizes and your deck clearance.
It's very possible to get off the shelf pistons that will put your actual compression ratio where it "needs to be" with the stock cam to run on 93 (or 91) octane. Those pistons will have some amount of dish in them to get to the needed clearance volume.

Welcome to the fold.... looking forward to hearing more about your plans.

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