Family car that I have owned for 33 years and purchased from my parents who had it since it was 1 1/2 years old. Meaning that I don't believe any work was ever done on the engine.
Rebuilding '72 Cutlass Supreme 455. Crankshaft bearings are identified in engine from front as;
Upper half B B F C A
Lower half A B F C A
Note: F is the center main

In the service manual it notes:
No letter --std
Letter "A" - .0005
Letter "B" - .0010
Letter "C" - .0015
Manual also states "It is possible to have more than one bearing size in the same engine".

I want to purchase new bearings, and they are sold as Std, .010", .020", .030" undersize.
Do I assume that what I need are Std and the fact that there are A, B and C's in the engine is that the engine builder measured the crank when building and had these "tighter" tolerance bearings to use when building? Do I also assume that the difference on the front of the engine with the B upper and A lower was a mistake by the builder grabbing the wrong A or B? Note, the A is the lower bearing at the front of the engine with no groove.

Thanks for any assistance!!

 

Yes. You will not find the select fit bearings in the aftermarket. They were used by the factory to try and get perfect clearances.

But what you really need to do is accurately measure your crankshaft before you order anything.

 

^^^ X2 ^^^

What is the condition of the bearings and the crankshaft? How many mies?

 

The crank diameters measure from 2.9988 - 2.9995". Book specification shows 2.9993 - 3.0003". Some of my measurements are under spec., but could be my measuring. I'll gage block the micrometer and do again. I seem confident however that I'm not out 0.010", indicating I would need undersized versus STD. When assembling I'll use Plastigauge to confirm. Thanks for your help!

 

Car has 114,000 miles, they are worn, but not bad. I want to change though, so I don't have to pull engine ever again!

 

I’ve seen factory original mains marked .0005, .001 I’ll post pics of them. the factory also had to have bearings that were +/- on the outside diameter of the shells to compensate for main bearing bores that were over and under the minimums. the letters could be the OD over/under unless it says it’s the ID.

I would measure each your main bore housings to see what you have.

 

Thanks very much for your help. Plan to measure everything this week.