/5 crankshaft interchangeability

Discussion in 'Airheads' started by Jo-Nathan, Oct 16, 2012.

  1. Jo-Nathan

    Jo-Nathan Been here awhile

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    So I fried a rod bearing in my 73 R75/5 back in July and it scored up the crank a little. It's my understanding that /5's of all displacements used the same stroke. Does that mean I can us a crank from any /5 (R60 cranks are easier to find and cheaper)?
    Thanks!
    #1
  2. brocktoon

    brocktoon Been here awhile

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    Yes. RealOEM.com shows that the part number for the crankshaft is interchangeable for all /5s and up through the /6s.

    I had to change the crankshaft out on my bike, and ended up going through 3-4 used cranks before I found one that fit, had good journals, everything within spec, fit into my crankcase and turned smoothly without binding. The color codes BMW used for the journals (red and blue) are a good start to get an idea for fit, but a micrometer will be invaluable in determining that nothing is out of round -- particularly on a crank of unknown origin.
    #2
  3. 983Bob

    983Bob Adventurer

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    The old part# for the 500/600 crank was 1121 1255 290 and the 750 was 11211259 856 including early /6 [to 76] which used the longer and stronger 10 mm bolts. The 900 part# was 1121 1262 450. The bearings are all dimensionally the same, but the balance factor will be a bit different. Since the factory now only has one part# for them all, I doubt that it will make much difference. Note that the /6 block was strengthened at the front which included a smaller opening, so an early crank [/5] will not fit through that opening, but for a /5 block, all the above cranks will fit.
    Hope this helps
    Bob
    #3
  4. mattcfish

    mattcfish R90X

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    +1. Use a good used R100 crank and a post 75 flywheel for the larger bolts. I bought a couple off eBay for cheap that were perfect replacements.
    #4
  5. Kt-88

    Kt-88 I like everything.

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    If you don't fix this I'm going to make you sell it to me. You're better off selling it actually.
    #5
  6. Jo-Nathan

    Jo-Nathan Been here awhile

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    What do the color codes mean? Where are they?

    RIghttt.....
    #6
  7. brocktoon

    brocktoon Been here awhile

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    The color codes are used to match crankshaft to crankcase bearing sizes. The front and rear main bearings in the crankcase should have a red or blue marking around their diameter, though they may have faded with time. You'll want to measure the I.D. of the crankcase bearing shells and compare them against the published BMW specs. If they're scored, out of round, or worn beyond spec you'll want to replace them.

    Similarly the crankshaft you buy should have color markings at the flywheel and alternator end. Again, measure their O.D. and compare them against BMW's specs for that color.

    Subtract the crankcase bearing shell's I.D. from the O.D. of its corresponding crankshaft journal. You're looking difference of 0.0028 to 0.0051 inches at room temperature. That gives enough radial clearance for oil. If you're within that range, and the crankshaft turns freely, you've got a winner.

    Here's some more info from Airhead guru Oak Okleshen on the subject:

    #7
  8. Kt-88

    Kt-88 I like everything.

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    Might be time to buy a known good r100 engine and stuff it in there.
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  9. pommie john

    pommie john Long timer

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    When I replaced the crank in my airhead racer I thought it was sensible to replace the bearings too so I took it to my local independent workshop who I've found to be very good.
    They told me they use red bearings in every one! They claimed they had never seen a crank that needed any other size!
    #9
  10. mattcfish

    mattcfish R90X

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    The shells will have the part number stamped on the outer serface, so you dfon't need to know what color they are. Check the Max BMW parts fische to identify. Most cranks have the red shells
    Most used cranks I've bought off eBay had the rods still attached. If the crank journal surface looks smooth and unscored, then it is most likely good. These cranks are over built and hardened. If they don't turn freely, they've probably been mishandled after removal, or the engine had a major seisure. Again, if the rods come with it and are themselves in good shape, you've probably got a winner.
    #10
  11. Boxer Metal

    Boxer Metal Mad Scientist

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    I have seen where the post 74 cranks will not clear the inside of the case on the earlier models. R60's have the same stroke, use the same bearing but have different counter weights.
    #11