GM undersize and oversize bearings. Original

[Lynn]

Tore down a 72 Chevy truck motor this evening. 3970014 four bolt block dated June of 72. Never been apart. Steel shim head gaskets still in place, measure about .015 thickness. Pistons .025 in the hole. Still standard bore, and not enough ridge to warrant a reamer to remove pistons.

All the bearings are dated April or May 1972. All the rod bearings say "GM Clevite 77" and are marked ".0005 OS"

All the mains have "GM AT-20" and some are marked ".0006 US" and some are marked ".0012 US"

Obviously, OS is oversize, and US is undersize.

Once all the rods and pistons were removed. The crank spun very freely, so I know it is straight. None of the bearings are worn badly, but there are some of the usual bits of trash imbedded, so I don't want to re use them. The crank is very nice, and will take almost no polishing with 1200 grit to clean up. Not a groove anywhere.

So, I don't want to have the crank ground. Don't know that anyone could improve on what I have.

I did keep track of which mains got the .0006 US and the .0012 US. Even if I didn't, a mic on the crank would tell me.
Seems to me one reason this engine made it all these years (cam went flat on 3 lobes and the timing chain finally broke... hmm, maybe that zinc in the oil problem doesn't just apply to the hp models?) was the attention to detail for the initial build.

Anyone make bearings in those dimensions any longer?

[Lynn]

Anyone, anyone... Bueller?

[marxjunk]

ive seen it in the past..ive even seen half of a oversized and a standard bearing used on one journal.

if get some standard bearings and plasti gauge them and see where you really are...follow the directions its kind of a lost art...

[Lynn]

Sure enough Mark. Just looked a little closer at the mains. One set is .0012 US lower shell, and .0006 US on the upper.

The rods are all identical, .0005 OS.

[bbbentley]

For several years of buying and tearing down Chevy engines, I had never seen undersize bearings in one from the factory. I was in denial. The General would never let one out the door with anything but Standard. Undersize was in Mopars, my buddies showed me. I would say, that is something Mopar does, Chevy doesn't. Then, I buy a 'Virgin' '72 350 sb one day and, voila, undersize rods. All .010 under. From that day, I never say never.

[bigsixman]

I had a 3.48" steel 350 crank back in the 80s that I bought from a friend that was .009" under. It was stamped .009 and the Chevrolet parts book showed a factory bearing for it.

I am unware of the original application for the crank, but it was steel and not tuffrided.