Strength of the 2.7 vs 2.5 bottomend?

[Hallen]

If the crank shaft doesn't have score marks, you need to measure each journal to check for wear before fitting new bearings. That is several measurements with a micrometer around and across each journal. If worn, the crank can be re-ground and over sized bearings used.

Exactly. Make sure the oil passages are clear.

There are three different bearing sizes that you can use. The cranks can be ground down to two undersizes. If you do that, I would suggest aiming for the lower end of the size for whichever undersize you end up doing. A little loose on the bearing (within spec) is better than tight.

You'll find the double oversize bearings for the rod journal a biotch to find in a complete set though. If you have to buy them separately, they run about $240 where a kit with all the bearings runs about $50 for the standard and single oversize. I guess not a lot of people go with the double size. A used crank will run you anywhere between 100 and 250 depending on condition, etc.

[nando]

grinding down BMW cranks is not worth it at all

you will have to have the journals nitride hardened again, which isn't cheap. and then having to deal with oversized bearings and other nonsense.

it's far easier and cheaper to find another crank in good shape.

[Hallen]

grinding down BMW cranks is not worth it at all

you will have to have the journals nitride hardened again, which isn't cheap. and then having to deal with oversized bearings and other nonsense.

it's far easier and cheaper to find another crank in good shape.

I would agree... unless your crank is already ground down.

Also, the hardening goes down a couple of 10ths so you aren't going to need to do the nitride hardening again unless you want to. Why else would they even offer oversized bearings? The factory couldn't expect people to reharden the crank. If there is something in the manual that I missed, which is altogether possible, I'll be more than happy to be corrected here. Knowing BMW, if the crank had to be rehardened, they wouldn't even give specs; they'd just tell you to buy a new crank.

[nando]

maybe a few ten thousanths, but not a few tenths of an inch

the people I've talked to about BMW cranks (like Jim from MM) warned that even polishing too deeply could take out the hardened layer, so I really doubt grinding to an oversized spec would leave anything behind.