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Old 07-05-2008, 10:40 AM
ScoTFrenzel ScoTFrenzel is offline
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One thing about rings and compression tests.
Remember the aluminum piston, when cold, has additional clearance to the cylinder wall. This allows the piston to rock, unseating the rings. Oil squirted in helps seal the minor leaks so compression goes up.
Commonly done on airplane engines.

This is the reason that the newer car pistons are of an alloy that expands little and the piston clearances are tight at 2.5 thou total. Less ring blow by for less smog. Older forged pistons set up around 6 thou. Think what happens when a piston and cylinder gets worn. It is not just piston slap.
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