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Thread: Cut crank integrity

  1. #1
    Registered User
    Join Date
    Apr 2020
    Charleston, SC

    Cut crank integrity

    Hey all, hope everyone is well and healthy. As Iím digging deeper into my motor (1976 R90/6) I noticed the nose of the crank is a bit deformed. Maybe where a puller had slipped off center. After I pulled the connecting rods I also noticed the big end rod bearings are oversized by .5mm. Iíve spoken with multiple friends on what Iíve found and I received two answers. First being that a cut airhead crank significantly loses its integrity and could fail. Or, that it is totally ok and there is no worry of it failing despite what has been done. Iíd like to reach out to yíall and get some advise on the matter. Since the nose is boogered up a little Iím inclined to purchase a used crank (one in better condition) but Iím just not sure if itís necessary and worth the extra money. Any opinion is greatly appreciated. Trying to get a feel on what an experienced owner would do if it were theirs. Thanks yaíll.

  2. #2
    Sir Darby Darryl Cainey's Avatar
    Join Date
    Mar 2003
    Fenwick, Ontario Canada
    A few pictures of the nose damage would help to make an assessment.

    If done right oversize bearings should not be a problem.
    Ambassador Emeritus BMW MOA Ontario Canada
    Niagara Riders
    Knights of the Roundel #333
    1977 R100RS, (Retired) 1993 R100GS

  3. #3
    Registered User
    Join Date
    Apr 2020
    Charleston, SC
    Thanks Darryl, Iíll get a picture loaded up today and I appreciate the opinion. Ted from the Beemer Shop told me he has seen under sized crank journals in the past but they are rare and a tell if the crank was turned would be to look at the counterweights because they need to be removed to properly turn the crank. Does anyone know of any other things to look for?

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