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Old 11-11-2006, 05:11 PM
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Compression height question

What would be the smallest compression height I could go with on a street motor with forged pistons, cast crank, and I-beam rods. I'm building a bbc and I need to get the block decked. I'm afraid if I take to much off Im going to have problems. Compression height for the block is 1.665 and the pistons I want to use have a compression height of 1.645 which gives me a .020 window. If I needed to take .0050 off of the block would the .015 left over be ok? This motor will probably never see more than 6500 rpms. Would a thicker head gasket be a safe way to correct this?

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Old 11-11-2006, 08:09 PM
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The ONLY way to get it right is to mock up the assembly in the block you are using. Do no. 1,7,2,8. holes. Then calculate the amount to be removed. The average shop can usually get it within about + -.002". I use masking tape on the ring lands to center the pistons..

GM block specs and your actual block are two entirely different things..
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Old 11-11-2006, 08:54 PM
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Like bob said ,but also you need to take in acct for piston to head clearance.
Sometimes you have to give a little on the deck height to keep the piston to head clearance in the safety zone.
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Old 11-12-2006, 01:15 AM
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What I'm concerned about is a problem that is undetectable with static engine parts. Component flex and bearing tolerances are different at 180 degrees and 6000 rpm than they are at stand still. What is considered safe?
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Old 11-12-2006, 06:04 AM
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Shoot for .040 total piston to deck with gasket installed. Basic stuff..
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Old 11-12-2006, 06:28 AM
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Quote:
Originally Posted by DaSouthWon
What I'm concerned about is a problem that is undetectable with static engine parts. Component flex and bearing tolerances are different at 180 degrees and 6000 rpm than they are at stand still. What is considered safe?
You made the statement of having .020 already in the assembly, and you want too take off .005. I would only take off what is actually needed to give the heads their truing, then shim back to give you that total of 0.020 clearance. 0.020" is just right on the margin anyway for heat expansion and mechanical flexing of the rotating assembly. Good luck n the build.
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Old 11-12-2006, 06:25 PM
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Quote:
Originally Posted by DaSouthWon
Would a thicker head gasket be a safe way to correct this?

A thicker head gasket could compensate for too much decking. But not the preferred way to go.
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Old 11-12-2006, 09:41 PM
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Mock up the short block on four corners like Bob said and measure the deck height. Let's say you have a piston to deck measurement of 0.025". Choose the head gaskets you will run so you'll know the compressed thickness. Let's say the compressed thickness is 0.041". You'll want to set the quench (piston crown to underside of head clearance) at somewhere between 0.040" and 0.045" for a BB because you have a heavier reciprocating assembly which will flex a little more and a larger bore so the piston will rock in the bore a little more. I use I use 0.035" to 0.040" on small blocks that I build. If you're going to shoot for 0.045", you would tell the machinist to take off 0.021" so that the piston is down in the bore 0.004". This clearance added to the thickness of the gasket will give you the 0.045" clearance you were shooting for.
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