478M Loosening Rod Bolts

[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...

[FetchMeAPepsi]

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.

[1972RedNeck]

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.

[Mike Caldwell]

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.

[1972RedNeck]

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.

[FetchMeAPepsi]

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.

[Mike Caldwell]

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?

[FetchMeAPepsi]

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.

[Mike Caldwell]

Add to the crank question -- could something have caused it to go out of balance???

[1972RedNeck]

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. :ahhhh:

[Mike Caldwell]

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

[1972RedNeck]

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]

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]

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. :thumbsup:

[1972RedNeck]

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

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

[bluestreak1854]

Redneck sir, I have a 478M out of a 8000 COE truck that only has 8696 miles on it. It has a Allison 6 speed auto transmission and I have all the mountings, radiator, shifter, and short drive line, everything to install it. I also have a 305V61 Clark 5 speed and a 6041 4 speed brownlite with shifter and pts if your interested.

[1972RedNeck]

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

That is definitely something to check.

Also, only the 379, 432, and 478 have externally balanced flywheels. The 401 should be the same crank as a 305 and 351.

[1972RedNeck]

Redneck sir, I have a 478M out of a 8000 COE truck that only has 8696 miles on it. It has a Allison 6 speed auto transmission and I have all the mountings, radiator, shifter, and short drive line, everything to install it. I also have a 305V61 Clark 5 speed and a 6041 4 speed brownlite with shifter and pts if your interested.

I would be very interested. Do you ever travel up towards Montana? Texas is a long ways but if we could meet somewhere in the middle...

[1972RedNeck]

are the mopar rods the same length?

I doubt it but I don't know. Due to the increased stroke, they will need to be shorter, but it doesn't mater as custom pistons will be needed anyway.

[Mike Caldwell]

Redneck sir, I have a 478M out of a 8000 COE truck that only has 8696 miles on it. It has a Allison 6 speed auto transmission and I have all the mountings, radiator, shifter, and short drive line, everything to install it. I also have a 305V61 Clark 5 speed and a 6041 4 speed brownlite with shifter and pts if your interested.

How much do you want for the 478? I've got family in Lubbock and may be down that way this summer and sure would like a good running 478.

PM me with details!

[1972RedNeck]

How much do you want for the 478? I've got family in Lubbock and may be down that way this summer and sure would like a good running 478.

PM me with details!

Hey, I got first dibbs...

[Mike Caldwell]

Hey, I got first dibbs...

Yep, but if you pass I want to be second in line:thumbsup: