Broken piston - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 01-23-2014, 09:37 AM Thread Starter
 
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Broken piston

1965 389. After installing a comp cams 262H, tripower heads and tripower, stock distributor with petronix. I broke #5 piston. replaced piston. drove 75 miles and broke same one again. Was told i'm running too much timing. What would be good total and initial timing?
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post #2 of 6 (permalink) Old 01-23-2014, 09:49 AM
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You are running too much compression, probably. Stock flat dished pistons? If so, the Comp cams 262H is not the cam to run. It is an excellent cam if you are running dished pistons in this engine, however. It also sounds like cast pistons. Did you break a piston ring land? Please post details about the engine so that we can assist: rebuilt or not, factory or replacement pistons, etc.
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post #3 of 6 (permalink) Old 01-23-2014, 04:54 PM
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I'd start with total timing (2500 rpm, vacuum advance disconnected and hoses plugged) at 34 degrees. If you're already in that neighborhood, then I'll bet your problem is way too much compression. That's assuming your engine was assembled correctly (cam degreed properly, timing set indexed properly, etc)

Do you have the pieces? Can you see any evidence of piston/valve contact?

Bear

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post #4 of 6 (permalink) Old 01-23-2014, 06:28 PM
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Good point with the valve/piston contact. If the wrong pistons were used, a real possibility. We need more info!!!
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post #5 of 6 (permalink) Old 01-27-2014, 02:26 PM
 
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#5 piston eh , that's the same piston that went on my sons 68 GTO 400.

Engine had maybe 300 miles on it since the PO had installed a new cam.

As bear stated, we assumed the cam was degreed properly and timing set indexed properly.......... man did we assume wrong

Bill
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post #6 of 6 (permalink) Old 01-27-2014, 04:21 PM
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We don't know if he hit a valve or broke a land. Until he posts again with some valid information, we're chasing our tails here. It could be mechanical interference, detonation, or assembly issues. No way of telling.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO General Discussion

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