Rod Bearing-follow-up

[DollyDodge]

I determined it was a connecting rod bearing that was causing the engine knock in my B-2-C. It was cylinder 3. I pulled the pan and took the rod cap off. The bearing was spun, and there is some marks on the crank. The bearing says on the back side

9185

Fm

7M

030US

Is the 030US refering to a .030 undersized bearing? If so I am a bit confused. As far as I know the crank was never turned. In 1973 we put new bearings in, but did nothing to the crank. I was just helping and didn't know what sized bearings were used.

Any suggestions on how to proceed (besides rebuilding-that will come when I get some funds saved up).

[MBF]

Depending on how marked up the crank is you may try cleaning it up w crocus cloth and then using a mic to see if its truly round or how far out it is. That will determine the next step which is to see if you can size and find a bearing

[Merle Coggins]

The only way to be certain is to measure the crank journal with a micrometer to determine the size. When I was in High School I rebuilt a 383 that I was sure had never been apart and found the rod journals to be .010" undersize. I then found one of the crank counterweights was stamped R010. My shop teacher told me that sometimes cranks get ground undersize at the factory if they can't get them smooth and round at the nominal size. It made sense to me at the time.

You mention marks on the crank. How bad is it? Can they be polished out? How does the connecting rod look? Did it get cooked, blackened?

Merle

[TodFitch]

...snip... As far as I know the crank was never turned. In 1973 we put new bearings in, but did nothing to the crank. I was just helping and didn't know what sized bearings were used. ...snip...

At least in 1933 there might be some code characters before and after the engine number that indicate if the engine came from the factory with undersized crank.

Maybe by the 1940s they had their manufacturing tolerances improved to the point where everything came from the factory standard sized. Or maybe not.

I don't have a truck manual but this evening I'll try to remember to look at the 1946-54 Plymouth factory service manual to see if they were still doing that non-standard sizing for cars in the 1940s.

[John-T-53]

Do what Merle says and check the crank journal for roundness. If its round and can be cleaned up sufficiently, you may be able to put in a new bearing to get you on the road for now. Check the crank counterweights for any machining stampings or machine shop paint marks in case it's been ground. Check the bearing clearance with plastigage.

If the journal is even moderately grooved or burred, I'd be skeptical about putting it into service again without taking it to a machine shop.

Usually engine bearings are resizable. The resized rod bearings on my flathead were stamped in ink ".010".

Fm = Federal Mogul

Edited October 13, 2009 by John-T-53

[grey beard]

A few years ago there was a post on this fourm about factory engines being found with one or more odd-sized rod journals. Methinks the bottom line was that the factory sometimes did this, rather than discard an otherwise-good crankshaft. It's not what any of us would want on a high-speed blueprinted engine, but it doesn't hurt a thing, as long as the journal isn't damaged.

The danger of a crank with an odd-=sized journal is that the average Joe looks at one bearing shell and reads "STD," and just naturally assumes the rest are all equal.

I have successfully cleaned up a rod journal where the bearing was spun, using journal-width crocus cloth strips and gasoline, and was able to get a good enough polish for a new insert to work. As Merle mentioned, this can only happen if the journal is still very concentric - not a bit out-of-round.

Let us know how you make out. Good Luck.

[DollyDodge]

Thanks for all the input. Sounds like I need to get myself a micrometer or find one to borrow. The marks on the journal don't look too bad. If I run my fingernail over them I can feel them, but I am sure I can clean them up. Worth a try any way. The Rod looks fine. As soon as it started knocking in 1979 my brother parked it. I started it a couple of weeks ago but didn't let it run long. Thanks again.

[Mike@DRH]

The crankshaft won't be your only worry.

Almost always when you spin a bearing the connecting rod I.D. will be to large even if it only ran for what you think is a very short time. You can look at it all day long and it may appear to be fine but you should never put any rod with a spun bearing back in a motor without first measuring it.

If you have as little as .001" oversize ID the rod will not grip the bearing properly and probably fail in short order. The only thing that keeps the bearing from spinning is the clamping force of the rod when torqued properly so if the ID is to large by even .001" it will fail again.

Mike

[Merle Coggins]

Actually, there should be a little tab on one end of the bearing shell, that fits into a notch in the rod, that keeps it from spinning when torqued up. However it's not strong enough to hold if the bearing seizes to the crank and tries to spin.

But yes, you should also check the rod too. Be sure it's smooth and not worn. But you'd need an inside mic to measure that and they're harder to find than an outside mic. You may find a shap gauge that will work for measuring the rod bore.

Merle

[DollyDodge]

thanks again for the info. I think what I have is a mess )

[Mike@DRH]

Actually, there should be a little tab on one end of the bearing shell, that fits into a notch in the rod, that keeps it from spinning when torqued up. However it's not strong enough to hold if the bearing seizes to the crank and tries to spin.

The tab is only on the bearing for alighnment during assy and has nothing to do with keeping it from spinning.

It's all about crush, meaning the bearing is bigger than the ID on the rod. No crush = no grip on the bearing and it will spin and shear the tangs off.

Mike