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Old 12-21-2014, 04:02 PM   #1
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Default Piston to Head Gap

Hi guys,

Rebuilding a race motor (high compression Nat. Aspirated) that I picked up as a spare. The engine was built by a guy here in Australia with a good reputation, and was a quick motor on the track.

It has some interesting stuff in it, but the most interesting thing we found was how little quench the car was set up with. Measured at the pin, it had less than .020" clearance to the head (counting the .040" head gasket - so the Pistons were sitting around .020" above the deck). With everything at workshop temp (72 degrees F), there was around .010" of piston rock as well, meaning that under load the quench gap might have even been less.

There is evidence of the Pistons hitting the head, but not hard.

Everything I have read is that running a piston to head of less than around .036" with steel rods is asking for trouble... Is this also the case with BP engines? This engine had 30 hard racing hours on it and did not grenade... And frankly, I am surprised.

Engine has a stock crank in a nb8b bottom end bored to 85.5mm with Supertech 11:1 pistons.
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Old 12-21-2014, 05:07 PM   #2
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The "safe" guideline I've heard is 0.1mm per 1000rpm. Run a little closer if you once you have successfull test data.

The rods on BPs is no different than other engines.
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Old 12-21-2014, 06:31 PM   #3
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So my math makes that around .035" at 9,000rpm (for safety).

Pretty much in my ballpark. The figures I was working on were general figures often used on engines with less main bearing support of the crank than a BP... Thought that may have made a difference.
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