Thin Head Gasket and Comprtession Ratio

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Matt69olds

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The most accurate way is to mock up a piston in the front of the block, and measure how far in the hole the piston top. Then put the SAME piston in the rear hole on the same bank, record the distance again. Measuring that way will allow your machinist to correct any difference in deck height front to rear. Then repeat the procedure on the other bank.

The reason you want to use the same piston is to eliminate any slight difference in connecting rod length, or piston height. And the reason for repeating on the other bank is to ensure things measure the same front to back, and side to side.

I am pretty good friends with a guy who use to run a performance machine shop, he said as long as nobody has had their hands in a engine, the factory blueprinting is usually pretty consistent and accurate. It’s the fast “rebuilders” kind of shops that do the sloppiest machine work. The best machining equipment is only as good as the guy using it.
 
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Ronno6

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The most accurate way is to mock up a piston in the front of the block, and measure how far in the hole the piston top. Then put the SAME piston in the rear hole on the same bank, record the distance again. Measuring that way will allow your machinist to correct any difference in deck height front to rear. Then repeat the procedure on the other bank.

The reason you want to use the same piston is to eliminate any slight difference in connecting rod length, or piston height. And the reason for repeating on the other bank is to ensure things measure the same front to back, and side to side.

That is an interesting concept.
I would have thought that you would want to mock up and measure using the actual piston/rod assemblies that will ultimately be installed in those cylinders-for the very reason you cite.
Measure using only one and machine based on that one, the dimensional tolerances of other pistons may not jibe with the one used. Hence, their fit in their respective bores may not be consistent. as I have all 8 slugs in their bores, I will measure all 8 and machine to the tallest piston.
Just my $.02.... But, then again, if I knew, I wouldn't be asking...........

In my case, should I look for "taller" pistons, with the 1.56" compression height in order to minimize deck removal??
 
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Matt69olds

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I’ll have to go back and check my notes. It seems to me decking the block to compensate for different deck heights based on each piston/rod combo couldean milling the block with a taper front to back. Of course, you could then measure center to center length of each rod, then the compression height of each piston, and try to switch things around to equalize things. Of course, then that might require the rotating assembly to need rebalanced, and then.......

When does it end? I guess that’s why professional engine builders change what they do for precision engine assembly. You could nit-pick things forever! I set my engine up with the pistons about 0.10, that leaves a little room for future repairs if needed
 

Ronno6

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I’ll have to go back and check my notes. It seems to me decking the block to compensate for different deck heights based on each piston/rod combo couldean milling the block with a taper front to back. Of course, you could then measure center to center length of each rod, then the compression height of each piston, and try to switch things around to equalize things. Of course, then that might require the rotating assembly to need rebalanced, and then.......

When does it end? I guess that’s why professional engine builders change what they do for precision engine assembly. You could nit-pick things forever! I set my engine up with the pistons about 0.10, that leaves a little room for future repairs if needed

Things would best be machined square and flat to a dimension based on the tallest piston.
I think that .010 is wise, as the proper squish can be achieved with a suitable thickness head gasket.
This would require smooth deck. I am not sure mine is currently acceptable. I have found Sealed Power pistons which have a compression height of 1.565. Those would reduce my depth to .015". Not bad, but they are costly.....
 

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All this may be moot....what C/R can I run with aluminum heads and still use 87 octane gas?
 

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The short block has been disassembled. (Darn hard to get oil pan off after siliconing the silicine gasket....)
If I have .030 shaved from the decks, the resulting .010 piston to deck height will yield about 9.875:1 C/R with the .041 gaskets I have.
That will have a quench thickness of .051"

How will that fair with 87 octane pump gas??

If I set things for the maximum recommended squish, .042" the C/R will be 10.1:1....
 
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Ronno6

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I'm gonna try to have .012 removed from the deck and use .015" shim gaskets.
If the decks do not clean up to the required 30Ra surface finish at .012", then I can go to .022" and use the Fel-Pro 1042-026 which is .026" thick. Those are more expensive but will work with surface finishes of 60Ra or better.
Those combinations will keep me in the .043"-.044" quench range and 10:1 compression.
If I need to run mid or hi octane gas, so be it.
I just don't want to leave any HP on the table with the parts I have chosen.
 

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I think you may be alright underneath 10:1 ESPECIALLY since quench seems to be a concern. I believe you are well-informed and are making sound decisions.
I am unaware of the operating elevation above sea level...which plays an important role so i've said it over and over.
Good luck!
 

Matt69olds

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Aluminum heads help a lot, as would a good cooling system and some kind of outside air induction, that being either a functional cowl hood, or the factory cold air system on the core support. The cooler the air, the better as far as detonation is concerned.

I’m at a little over 11 to one on my Olds engine, and just as an experiment, put a half tank of 89 octane. Obviously I didn’t beat on it, but normal driving I never heard the first rattle. The car runs the same times on premium pump gas as it does with a little race gas. That tells me the engine isn’t octane limited. When I’m pushing that little blue button I make sure there is plenty of good gas in the tank. Race gas is expensive, but not nearly as expensive as shattered pistons!! I think if you pay attention to your tune, and make sure the rest of the engine and truck is up to standards 10 to 1 compression is safe.
 

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When I added Blueprint aluminum heads to my SBC crate the 64cc chambers increased compression, but I also used the Mahle 5746 gasket (.025") to get just a little more compression. Those gaskets were recommended by several guys on the hotrodders board, and they seems to be working well.

With my stock crate pistons, 64cc heads and thinner gasket, I'm probably only at about 9.4 compression, so it runs on any type of gas. However, its a lot better than the original ~8.2 compression of the GM base crate engine.


Bruce
 

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This exercise has taught me a lot, but it comes at a price......1 piece pan gaskets are not cheap, and having the block decked necessitated new cam bearings and tanking the block again. Plus the head gaskets.....but, there was no sure way of knowing that or how much deck machining was going to be required til the crank,rod and piston wrer installed. Having it decked beforehand would have been guessing, or yielding dimensions that would have to be dealt with afterwards, possibly to a compromised result.
As it is, I think the best of all results will be achieved, at least from a specification standpoint.

ps....My city is listed as being at an altitude of 259'
 
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If the 1-piece pan gasket comes off without any damage, you can reuse it.
 

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