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Author Topic: Loose Wrist Pin In Rod  (Read 541 times)
69-er
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« on: April 22, 2013, 06:18:39 PM »

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 stock build with 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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Mike S
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« Reply #1 on: April 22, 2013, 06:32:13 PM »

 I suspect the ID of the rod end is oversize so the pin is able to work its way out.
Too bad the machine shops these days can't dip the rod ends in copper to take up the extra diameter like the factory did.

Mike
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z28z11
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« Reply #2 on: April 22, 2013, 08:55:33 PM »

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.

Likely not, considering the method shops use to change pistons - heat the pin end of the rod (with torch, or induction heating coils) until the pin gets loose enough to push out. Rods that worn obviously cannot be recovered, unless you bush them. If they are standard rods (not "pink" rods), that's probably not cost effective. "Pinks" came both as press fit pins and floaters: the biggest differences involved shot peening - pinks were peened, standards were not.

When I rebuilt my 302 in '75, I had the original rods bronze bushed. The shop that rebuilt them in Nashville cussed me when I picked them up - they noted that they expended many a drill bit attempting to drill the pin oiling holes - shot peening really works -

Regards,
Steve
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