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Thread: M30 engine rebuild bearing wear

  1. #1
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    M30 engine rebuild bearing wear

    Tearing down spare m30 for rebuild. Plan to replace rod bearings and possibly swap in forged pistons. I’d like to reuse mains but the wear seems iffy. Was planning to use plastigauge and reuse if clearance is ok. Anyone have expertise here? Also what do you think wear is a result of? My guess was dirty oil.

  2. #2
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    "Iffy"?
    Turn in your tools, man.
    Last edited by ross1; 06-11-2019 at 06:24 PM.

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  3. #3
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    “Iffy” is a very technical term for...ehhh I could spend the money but damn this is will add up quick.

    Point taken sir. I know I “should” replace them, just needed that extra push. I see Ireland engineering indicates there can be variances in crank size on m30, does m30b35 in e34 deviate from standard?

    Any brand should stay away from?
    Last edited by johnnylud3; 06-11-2019 at 07:35 PM.

  4. #4
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    Quote Originally Posted by johnnylud3 View Post
    “Iffy” is a very technical term for...ehhh I could spend the money but damn this is will add up quick.

    Point taken sir. I know I “should” replace them, just needed that extra push. I see Ireland engineering indicates there can be variances in crank size on m30, does m30b35 in e34 deviate from standard?

    Any brand should stay away from?


    Re-using those bearings is a fool's errand and false economy.
    Forged pistons imply that you may have some heavy duty use in mind. If you just want to assemble a cheap "grenade" motor to thrash to death then screw it back together as is and wait for the inevitable.

    You said rebuilding. "Rebuilding" implies, no clearly states, that everything has been restored to new specification. Anything less is simply repaired as needed. And yes, it does add up quickly, especially after a trip to the machinist.
    Crank should be inspected and probably at least polished. Hopefully clearances will remain within tolerance for std. bearings and it won't need grinding. Same goes for the rods and re-bush the small ends.


    Any reputable bearing brand is fine, I believe Kolbenschmidt is OE but doesn't need to be German. Don't order them until you know the finished size of the crank journals.

    Glad you got the message so suck it up buttercup, engine building is expensive.
    Good luck with whatever you decide.

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  5. #5
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    Anyone happen to have the specs for wear tolerances on bottom end of this engine? Seems to be void in the Bentley.

  6. #6
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    Do these help?


    Quote Originally Posted by 1990525i View Post
    the bees knees

  7. #7
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    Yesshhh yesshhh! Do you happen to have anything for cylinders? Also, seems there are 3 possible cranks used in m30b35 with varying main bearing size. When I pull up main bearings at parts suppliers websites, there is only one option for standard sized bearings. Did e34 535i only have one of these 3 cranks (red, yellow, green)?

  8. #8
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    Annotation 2019-06-26 001840.jpg

    Here. Not sure how to interpret 1) and 2).


    From a file in my car documents called "E34 Tolerances".
    Last edited by paperplane94; 06-28-2019 at 02:57 PM. Reason: Citation

    Quote Originally Posted by 1990525i View Post
    the bees knees

  9. #9
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    I believe there is only one crank journal size, the three different "colors" are to optimize clearances, compensating for minor (very) differences due to mnfg. tolerances. The entire range is only .0007"
    I skilled machinist can examine and measure your crank and tell you if it needs simple polishing(and hopefully remain in the "standard" range) or will need grinding in which case the crank is ground to suit the available bearings.

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  10. #10
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    Those bearings look pretty average for being used with I'm guessing well over 100k on them. Not bad at all. They could go for another 100k but while you're in there, might as well do them.

    I'm sure the engine wasn't knocking looking at the pics and was factory original before you took it apart so I would order new, standard size shells and button it up. I would do nothing to the crank journals. No spun bearings, no problem.

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