Piston Rock at TDC

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Jun 24, 2009 | 01:29 PM

the_merv

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From: The Beach...

Piston Rock at TDC

How much Piston Rock should there be at TDC?

I have a 6.0l Block, 4.000" Bore with Eagle 4.000" Crank & 6.125" Rods. I just pulled the Heads cause I have bad Rings, I'm wanting to make sure that I don't have excessive Piston Rock.

Jun 24, 2009 | 02:33 PM

DeltaT

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You'd have to measure the pistons and actual bore and then compare your numbers to the piston mfg's suggested fitment numbers for clearance. I always think that my piston rocks too much even when the clearance is set dead nuts on.

Jim

Jun 24, 2009 | 05:53 PM

the_merv

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I got a measurement on the Bores of a 4.003"-4.004"..so that may be a good reason for my problems..

Jul 14, 2009 | 11:56 PM

MAC4264

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What brand piston. Look at some of my old threads.

Jul 15, 2009 | 08:29 AM

-Joseph-

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The crown of most forged pistons is roughly .055" smaller (when cold) than the diameter at the piston skirt. This is intentionally machined by piston companies due to the top of the piston being in the line of fire and absorbing direct combustion heat. As the engine is warmed up this clearance tightens up significantly, and when running warm has very little piston to bore clearance. I don't think you could take it apart quickly enough to measure the actual running clearances though.

Unless the piston rocks 1/4" or more, cold-piston rock tells very little about what the clearances are.

Jul 15, 2009 | 11:39 AM

racer7088

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Quote:

Originally Posted by the_merv

Stock pistons cold rock about .020 ish as in if one side is pushed sideways it may go .010 out of the hole and then .010 in the hole when rocked the other way. Forged pistons have generally more rock when cold. Some of the wiseco and Diamond LS1 specific stuff is still only like .020-.025 where as some traditional forgings and skirts can be much more like .030-.040 even so it all depends. When hot these all rock much less.