478M Loosening Rod Bolts

[Mike Caldwell]

You trying to make me drool? What kind of torque would that make???

[1972RedNeck]

Originally Posted by Mike Caldwell

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.

[TJ's GMC]

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.

[James]

Originally Posted by 1972RedNeck

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).

[1972RedNeck]

Originally Posted by James

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...

[TJ's GMC]

I wanna see a ton of pictures and a video of how it runs if that's the case.

[1972RedNeck]

Originally Posted by TJ's GMC

I wanna see a ton of pictures and a video of how it runs if that's the case.

All in due time.

[ilvracn]

are the mopar rods the same length?

[bluestreak1854]

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.

[bluestreak1854]

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