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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old January 23rd, 2017, 12:41 AM
1972RedNeck 1972RedNeck is offline
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Default 478M Loosening Rod Bolts

So I have a line on the front half of a '74 GMC 8500 with a 478M, five speed, and four speed OD brownie. The engine has had multiple fresh rebuilds. Apparently, after the first rebuild, one of the rod caps came loose. They pulled the pan, put a new bearing in it, and torqued it back to spec. It came loose again, and another time.

It was torn completely down and all new rods and pistons were put in. And it shortly thereafter developed a rod knock and was parked.

What do you all think could be the problem with it? The price is right, but I don't really need something that I can't fix either...
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Old January 23rd, 2017, 01:57 AM
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FetchMeAPepsi FetchMeAPepsi is offline
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Default Re: 478M Loosening Rod Bolts

My guess is that the rod cap was bad (worn). Worst case if that's the only problem I'd say you're buying a new rod and cap. If the price is right I'd buy it anyway just to have the extra parts.
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Some people are like slinkys. Not worth much but funny as heck when pushed down stairs.
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Old January 23rd, 2017, 02:00 AM
1972RedNeck 1972RedNeck is offline
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Default Re: 478M Loosening Rod Bolts

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Originally Posted by FetchMeAPepsi View Post
My guess is that the rod cap was bad (worn). Worst case if that's the only problem I'd say you're buying a new rod and cap. If the price is right I'd buy it anyway just to have the extra parts.
After the first couple tries, new rods were put in and it still did it.
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Old January 23rd, 2017, 02:04 AM
Mike Caldwell Mike Caldwell is offline
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Default Re: 478M Loosening Rod Bolts

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Originally Posted by 1972RedNeck View Post
After the first couple tries, new rods were put in and it still did it.
But were the bearing caps replaced? I'm betting they didn't.

Check the clearance on the bearing and I'll bet it's looser than the others.
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Old January 23rd, 2017, 02:05 AM
1972RedNeck 1972RedNeck is offline
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Default Re: 478M Loosening Rod Bolts

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Originally Posted by Mike Caldwell View Post
But were the bearing caps replaced? I'm betting they didn't.

Check the clearance on the bearing and I'll bet it's looser than the others.
They did. All the old rods, caps, and pistons are on the floorboard.
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Old January 23rd, 2017, 02:06 AM
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Default Re: 478M Loosening Rod Bolts

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Originally Posted by Mike Caldwell View Post
But were the bearing caps replaced? I'm betting they didn't.

Check the clearance on the bearing and I'll bet it's looser than the others.
Yup, there's only two things at work there and if they replaced the rods and didn't replace the caps, that's the problem.
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Some people are like slinkys. Not worth much but funny as heck when pushed down stairs.
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Old January 23rd, 2017, 02:12 AM
Mike Caldwell Mike Caldwell is offline
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Default Re: 478M Loosening Rod Bolts

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Originally Posted by 1972RedNeck View Post
They did. All the old rods, caps, and pistons are on the floorboard.
Did they reuse the bolts? (been too long since I've been inside one -- do they use bolts or studs?)

Does the block need to be line bored?

Just had another thought -- could the crank be bent/twisted so it causes vibration in that cylinder?
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Old January 23rd, 2017, 02:12 AM
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Default Re: 478M Loosening Rod Bolts

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Originally Posted by 1972RedNeck View Post
They did. All the old rods, caps, and pistons are on the floorboard.

Haha, we replied at the same time. So yeah, the rods and caps are both replaced and still the bearing came loose? AFAIK that's not possible. I'd buy that in a heartbeat and fix it myself.


EDIT: Ohh, mike could be right on the bent crank shaking it out.
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Some people are like slinkys. Not worth much but funny as heck when pushed down stairs.
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Old January 23rd, 2017, 02:39 AM
Mike Caldwell Mike Caldwell is offline
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Default Re: 478M Loosening Rod Bolts

Add to the crank question -- could something have caused it to go out of balance???
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Old January 23rd, 2017, 02:50 AM
1972RedNeck 1972RedNeck is offline
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Default Re: 478M Loosening Rod Bolts

Don't know. I guess I could buy it, offset grind the crank to 4.25" using BB Mopar rods and custom pistons and make a 532.
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