Can anyone explain what a stage 1 2 or 3 is with regard to crankshaft journals

[jjs2800cs]

Trying to figure out why there is listed in the specs a stage 1,2, and 3 as well as the original with regard to crank journals?

Also same situation with cylinder bores and piston diameters. What does A B and C mean as opposed to the original?

Thanks
jjs2800cs

 

[eriknetherlands]

Dunno exactly, no engine pro myself, but I just happen to have a set of pistons on my kitchen table.
Each piston is marked with its specific dimension. This one for instance is 88,96mm.

Taking a guess; bmw matched the pistons one the size of the bore?
Production always has tolerance, this is one to cope with that: did the bore turn out a bit large? Choose piston "C"

 

[E9madada]

Stage 1,2 & 3 are the measurements of the regrinds of the crank and associated bearing thickness.
Not sure what the A B C are.

 

[jjs2800cs]

The specs also list an A B and C for the cylinder diameter, so may as eriknetherlands suggests it has to do with matching them in a class when first assembled. My original old pistons just removed are stamped A3 with a minus sign. So I must have A pistons with a minus weight class, but have no idea what the 3 means. My new pistons are marked 85.97 (car is a 2800). Both measure around the same area on the skirt the exact same amount - 85.97mm. So I would guess my original motor had an A bore of 86mm and was fitted with matching A pistons. But the 3 is a mystery.

Anyone care to chime in?

Whats amazing is the original engine with high miles has all the bores measuring almost exactly 86mm. Anyone comment on the seemingly lack of wear? Am taking to a machine shop to get precise measurements. Hoping we can use a new set of standard pistons purchased many years ago. Otherwise new custom pistons are not cheap and neither is getting the block sleeved to receive standard size pistons.

Thanks
jjs2800cs

 

[m5bb]

Take this chart to your machinist and he will explain.
BMW allowed for rebuilds of their motors.
The bearing "sets" would be smaller than the originals allowing the crank journals to be ground down to get rid of a defect or out of round condition.
BMW M30 motors do not wear very much in the crank and rod journals and usually the original size bearings can be used on a first rebuild and be within tolerances.
Pistons are similar and allow for boring the block to fit a bigger piston to get rid of scoring and issues in the cylinders.
Aftermarket pistons are relatively easy to source and much cheaper than BMW OEM and probably better because of advances in piston design.

Gary