Recently I learned of two crank main bearing color codes that are not covered in the Clymers or BMW shop manuals I have. There is a very brief reference in Haynes, but no details. The colors are yellow and green and I believe they are for smaller cranks than the blue and red that are covered in all the parts and service books. I see the yellow and green main bearings are offered up for sale at Moto Bins, but not Max. I contacted MB and they have no info on their fitment. There must be a specifications chart that my search didn't turn up?
You'll find the yellow and green bearings in the /5 'fiche. The /5 cranks are normally the only ones that use bearings of this size. By the way, a /5 crank will not fit in a /6 on crankcase.
That is interesting. I just picked up a very low mile 78 R100 short block. The mains are smaller than blue or red, rod throws measure no wear and I am conviced this is a virgin short block. I wonder what is it that won't allow the /5 crank in a /6 block?
Umm, could you please explain why? The part number for the R75/5 and the early R75/6 (as well as the R90/6 and R90S) are the same according to my factory parts manual. The only difference with the cranks came with the change from 10mm to 11mm flywheel bolts at some point in 1975 as far as I'm aware.
The wheels on the /5 cranks are larger in diameter than the later cranks, as is the hole in the front of the case. You can take it from someone who has tried it that it won't go in. I knew a guy who went to the effort-at a race track!-of cutting away some of the case on his /7 motor to get it ready for his next race. Occasionally the mains on a later crank will dictate a smaller than blue bearing, but my experience is that the bearings rarely are the size they're supposed to be. You have to install them, let them cool, and measure them to determine their actual size.
I have had a 75 block around here for a while that I never could get the crank out of. When I looked closely, there are punch witness marks on it, where it appeared someone forced it in. Swing press? What you say about /5 cranks/blocks may finally explain the mystery. The 78 crank I have is clearly under the blue size, because it clunks around when inserted in a block with blue bearings.
Was BMW ever known to build engines with over sized, stage 1 etc. bearings? I have always wondered why they supply Stage 1, 2 and 3 bearings if no one is successfully turning cranks.
I've seen two motors-one in a monolever R80, and one in an R75/6-which had .25mm undersize rod journals from the factory. The R80 also had bronze bushings in the lifter bores. I doubt they would have done this on any of the larger motors, although that R80 has been running for years with a 1050 kit.
I finally realized today that my crank main bearing journals measure like stage 1 oversize. The red stage 1 is 59.73-59.74 and my rear main journal is 59.7408. The blue stage 1 is 59.721-59.730 and my front main journal is 59.707, a little under. This wouldn't bother me but I don't know for sure if this stage 1 sizing is the result of the factory or some crank grinder and destined for failure. The rods are standard and again the engine parts I have examined show very little wear and no evidence of anyone taking the engine apart before now. It is comforting, that you have seen evidence of factory installation of that type though. I think there are so few mainbearing failures that alot isn't known about what the factory did.
It would be unusual for the mains to be reground post-factory but not the rods since the rod bearings are the ones that fail when subjected to low oil pressure (or dirty oil), so I would bet on a factory undersize. Do you have any idea how long has this crank been in use?
I don't know how long it has been in service. All of the parts I have taken off the block show very little wear. The oil pump and rear cam bore are completely scratch free. This would indicate to me there has not been a bearing failure and think the crank and bearings are original. But who knows for sure?
Just some food for thought... I don't know shit about airheads (although I'm an experienced mechanic). (some of) The (Austrian built) Rotax snowmobile engines I've rebuilt have the usual "stanadard" then first, second oversize pistons and rings available. THEN, there is the color code. That is, the engine comes from the factory with a "standard" piston, but it may have green or yellow color coding. It signifies "as built" dimensions, or "size sorting" of cylinders and pistons. Takes the precision fit to the next level. This discussion about bearing sizes made me think if you guys had something similar going on. JFWIW
