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Old Apr 22nd, 13, 04:13 PM
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Default Loose Wrist Pin In Rod

What would cause a wrist pin to come loose in the rod? The engine developed a knock and upon tear down I found that the wrist pin had moved back against the cylinder wall and gouged two grooves in it. It's a factory 68 Firebird 400 with the exception of 9.5 pistons and a mild cam. The engine has 1,000 miles on it since a complete rebuild.

A little background:

There have been two sets of pistons installed recently. The first set was during the first rebuild, 7,000 miles ago. The second set was after the engine overheated and the engine rebuilt again. The overheating caused piston and cylinder scuffing on two cylinders. The majority of the damage was with the same cylinder as the loose pin. The adjacent cylinder also had a little bit less scuffing.

I'm thinking the overheating had something to do with it. I'm wondering if the machine shop should have detected a little bit less force required to remove the damaged piston. I'm sure if the overheating did in fact cause the loose pin, it should have been noticed then.

At this time, the pin is loose enough to be pushed out with light tapping.

Any ideas? Thanks!
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  #2  
Old Apr 22nd, 13, 06:12 PM
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Default Re: Loose Wrist Pin In Rod

Shops, as I remember, use a stove to heat the small rod end to cherry red, then insert the rod while hot, into the piston and press the pin in place using a small hydraulic press. If the tech wasn't watching the pressure gauge, he/she would never know the 'force' used to press the pin in place.

My thought is the Pontiac block, same cylinder on both rebuilds, or a faulty piston rod, as in overheated and has taken a different dimension since then, hence the looseness.

Maybe the professional engine builders will diverge their knowledge.
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Old Apr 22nd, 13, 08:00 PM
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Default Re: Loose Wrist Pin In Rod

Yeah as Everette said. I was reading the How to rebuild Chevy engine book today and there was something in it about rods getting damaged at the wrist pin if the engine was to overheat. It can happen according to the book, maybe from before the last engine build?
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