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GMC V6 and V12 Engines Engine repair and rebuilding

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  #11  
Old January 23rd, 2017, 02:52 AM
Mike Caldwell Mike Caldwell is offline
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Default Re: 478M Loosening Rod Bolts

You trying to make me drool? What kind of torque would that make???
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  #12  
Old January 23rd, 2017, 02:58 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
You trying to make me drool? What kind of torque would that make???
A good bit. That is the eventual plan for the 478 going into my '72 K20. It is going to take a few years savings for the machine work, pistons, and porting though.
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  #13  
Old January 23rd, 2017, 06:04 PM
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Default Re: 478M Loosening Rod Bolts

478's were known for crankshaft problems I believe...especially the early ones. Not to mention these v6's were horribly balanced to begin with. I'm betting that engine has been over revved most of its life.
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  #14  
Old January 24th, 2017, 12:05 AM
James James is online now
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Default Re: 478M Loosening Rod Bolts

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Originally Posted by 1972RedNeck View Post
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...
I wonder if they check the sizing of the new rods before they installed them. I would resized the rods to fit tighter on the bearings and maybe install a dowel pin in it to lock the bearing in place (like some of the racing engine mechanic do to their engine).
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  #15  
Old January 24th, 2017, 12:57 AM
1972RedNeck 1972RedNeck is offline
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Default Re: 478M Loosening Rod Bolts

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I wonder if they check the sizing of the new rods before they installed them. I would resized the rods to fit tighter on the bearings and maybe install a dowel pin in it to lock the bearing in place (like some of the racing engine mechanic do to their engine).
If any machine work gets done, the journals are getting turned down to fit BB Mopar rods...
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  #16  
Old January 24th, 2017, 03:55 AM
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Default Re: 478M Loosening Rod Bolts

I wanna see a ton of pictures and a video of how it runs if that's the case.
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  #17  
Old January 24th, 2017, 03:57 AM
1972RedNeck 1972RedNeck is offline
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Default Re: 478M Loosening Rod Bolts

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I wanna see a ton of pictures and a video of how it runs if that's the case.
All in due time.
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  #18  
Old January 25th, 2017, 04:49 PM
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Default Re: 478M Loosening Rod Bolts

are the mopar rods the same length?
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  #19  
Old January 29th, 2017, 06:03 PM
bluestreak1854 bluestreak1854 is offline
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Default Re: 478M Loosening Rod Bolts

Have you made sure you have the right flywheel and harmonic balancer? With the vibration loosening the rod nuts, it could be that the crank is not balanced as a result of having one of these incorrect.
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  #20  
Old January 29th, 2017, 07:20 PM
bluestreak1854 bluestreak1854 is offline
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Default Re: 478M Loosening Rod Bolts

the 401 and 478 have an externally balanced crankshaft-- that means that the flywheel and harmonic balancer have the counter weights on them-- if someone has the wrong flywheel or balancer on this engine it could be sending vibrations that are loosening the rod nuts-- but the vibration should be very noticeable
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