Rod & Main Bearing Clearances
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Rod & Main Bearing Clearances

  1. #1
    Senior Member Brendellajet's Avatar
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    Default Rod & Main Bearing Clearances

    Posting this for my brother

    BBC, assembling bottom end after a little mishap. Freshly polished crank. Rod bearings are at .0019. Acceptable limits I've read are .0020-.0025. Should he have the crank polished more? Hes already using the oversized bearings(+.0001).

    Are Mains ok in the .0025-.0030 range? Im not sure where he is at there, but Im sure he wants a confirmation...

    Thanks for the advice!

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  3. #2
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    a + bearing is a thicker bearing....aka less clearance. An X bearing is a thinner bearing....aka more clearance.

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    Senior Member Brendellajet's Avatar
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    Quote Originally Posted by Fiat48 View Post
    a + bearing is a thicker bearing....aka less clearance. An X bearing is a thinner bearing....aka more clearance.
    My bad, pretty sure he went with the X for more clearance.

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    Course there are (or will be) a lot of opinions. I say min .0025 and would rather see .003.
    The mains .003 also.
    .0035 on the mains if it is blown.
    You might want to be sure of the bearing you are checking the rod clearance with....because if it is .001 extra clearance bearing and you have .0019 with it.....you'd have less than a thousandths clearance with a "std' bearing......which shows something is not right (either rod size, crank size, or your method of measurement.)

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    steelcomp was here
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    IMO, if you intend on running a light weight synthetic after breaking it in, (5W-30) then .0020-.0025 on rods, and .0025-.0030 on mains (I'd kep them around .0025 if possible).
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    Some guy obnoxious001's Avatar
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    Quote Originally Posted by Fiat48 View Post
    You might want to be sure of the bearing you are checking the rod clearance with....because if it is .001 extra clearance bearing and you have .0019 with it.....you'd have less than a thousandths clearance with a "std' bearing......which shows something is not right (either rod size, crank size, or your method of measurement.)
    I agree, .001" with standard bearings brings up a red flag! Maybe he mistook .001 bearings for extra clearance?

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    Default Typo or mis information

    Quote Originally Posted by Brendellajet View Post
    Posting this for my brother

    BBC, assembling bottom end after a little mishap. Freshly polished crank. Rod bearings are at .0019. Acceptable limits I've read are .0020-.0025. Should he have the crank polished more? Hes already using the oversized bearings(+.0001).

    Are Mains ok in the .0025-.0030 range? Im not sure where he is at there, but Im sure he wants a confirmation...

    Thanks for the advice!
    I think this is a typo, but maybe it is what he meant to type. We know a tenth over aint available, but does anyone there really know what they're dealing with? Sorta raised a red flag to me...........Maybe post an actual part number.......Ray
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    Senior Member Brendellajet's Avatar
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    Quote Originally Posted by Moneypit View Post
    I think this is a typo, but maybe it is what he meant to type. We know a tenth over aint available, but does anyone there really know what they're dealing with? Sorta raised a red flag to me...........Maybe post an actual part number.......Ray
    It probably was a typo. I will check with him to see. He probably did go with the oversized bearing. So if he uses only half of the oversized bearing he should be good assuming his reading of .0019 is accurate?

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    Senior Member VAMI's Avatar
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    743hx adds.001 743h1 subtracts.001 check the the housing bore on the rods they could be on the small side,the size is 2.3247/2.3257,you can open the rods up to the high side.you need at least .0025 clearence.Try to get .003 on the mains.

  12. #10
    Senior Member MACHINEHEAD1's Avatar
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    Cant polish any journal to get clearance. It will end up barrel/hourglass/cone shaped. Dont know anyone that can grind a thou off either. Dont open up the big end either unless its below 2.3247. Do the bearing thing or find another grinder.

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