Airhead main bearing

Discussion in 'Airheads' started by DiabloADV, Apr 7, 2012.

  1. DiabloADV

    DiabloADV Semi-Occasional

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    [posted this at bmwmoa, but posting here as well...more traffic here...]

    Against good advice, I pulled my bottom end apart, as much out of curiousity as anything else. This bike ('70 60/5) has a questionable past, so I figgered I'd have some confidence going forward. One thing that led me to want to look at the rear main bearing was that the right con rod bearing was unusually worn. Since the rear main and right side con rod bearing share an oil channel....

    Well, lo and behold, the rear main bearing is scored up pretty good. The crank end journal is clean as can be, and right on factory spec. So, I'll replace that bearing. I tap it out after heating the engine case, correct?

    Is it possible to install the new rear bearing without the special tool shown in the shop manual? The Haynes manual sums up the procedure like this: Take it to the dealer.

    I don't wanna take it to the dealer. Any tips before I ruin something...?
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    #1
  2. PaulRS

    PaulRS Dutch fool

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    Note that those bearings come in different sizes, colourcoded.
    See if you can find some of that colourcode on the side of the bearing.
    It should be green, red , yellow or blue, iirc.

    If so, fit a new bearing with the same colourcode.

    If not, carefully measure the old bearing, fitted in the casing, and refit a new bearing one size bigger, and measure again.
    This is done by 'trail and error', if the new bearing doesn't meet the required play, fit another one.
    Or so I was told by an experienced BMW wrench.
    Those bearings tend to shrink a little when fitted.

    For the special tool, no need, heat the casing, freeze the bearing and carefully tap it in.

    Paul.
    #2
  3. bmwrench

    bmwrench Long timer

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    You'll need to get the case quite hot-around 350 F, preferably in an oven. The old bearing will tap oiut easily at that temperature. Let the case cool and polish the bore and clean thoroughly. You will need to fabricate some kind of tool to keep the bearing from going in too far; the factory tool is not much more than a piece of pipe with holes drilled for the thrust washer pins, and a piece that drops into it to stop the bearing. When you are ready to install the new bearing, again heat the case to 350. The bearing should drop in. The final piece of the puzzle is the the split in the bearing is supposed to locate at about 2:00 as viewed from the rear.
    #3
  4. DiabloADV

    DiabloADV Semi-Occasional

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    Ok, got it. I can make a drift for it out of PVC pipe, I think. But it sounds like I won't even need that...getting the depth right should be no problem since I have it torn totally down.
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  5. DiabloADV

    DiabloADV Semi-Occasional

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    The shop manual shows that procedure pretty clearly.

    Question...the various part numbers for these bearings have specs for undersize journals, correct? ie, there is a +.25, +50 and +75 for each of red and blue. My crank journal has not been machined and measures to stock spec. So...I need the stock size bearing.

    But...without remaining color code, how do I know if I have red or blue?
    #5
  6. bmwrench

    bmwrench Long timer

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    The /5s have 4 different sizes in each spec: Red, Blue, Green and Yellow, with red being the largest I.D. Examine the edge of the old bearing-you may still be able to see the original color code. You should still measure the journal in several places and check your measurements against a real BMW manual. I find that the bearings BMW provides don't neccesarily match their own specs, so after the bearing is installed, you'll have to check the final. I.D.

    I think you misunderstood the note about the bearing installation depth. If the case is at the proper temperature the bearing will fall into the case. That's why you need something to stop the bearing.
    #6
  7. DiabloADV

    DiabloADV Semi-Occasional

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    I nudged the two thrust spacers/bearings out (bearing still in the case). The edge of the main bearing is definitely red. I can also see how it needs to stop flush with the interior of the case. Since my case is empty, that should not be a problem to accomplish without the special tool.

    Interestingly...the shop manual says to install that rear main with the oil holes at noon and 6 -- with the bearing seam at 2 o'clock, as you mentioned. My rear main is turned nearly 90 degrees, with the oil holes at 10 and 4. So...it turned in the case. But, why wouldn't it? It's not pinned like the front main in its carrier.

    This lack of pinning in the back seems odd, or does the press-fit just normally keep it in place? if mine turned, that means it was binding on the crankshaft. I've got to figure out what blocked the oil flow...
    #7
  8. bmwrench

    bmwrench Long timer

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    The interference fit is in reality what keeps all bearings from moving (with any luck). The new bearing, unless you find an NOS one, will be slotted so oil hole location will be unimportant. Your bearing may have been located improperly when it left the factory, or may have been replaced sometime in the forty years since then.

    I am tempted to not pin the front main every time I replace one, but then I think about the possible consequences.
    #8
  9. Boxer Metal

    Boxer Metal Mad Scientist

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    I would like to see a picture of the bearing? It is super uncommon for something like this to happen. Pull the oild pump cover while you have the flywheel off and remove the inner oil pump rotor and make sure the key way on the end of the camshaft is not broken. That is more common on pre 74 models.
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  10. DiabloADV

    DiabloADV Semi-Occasional

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    Camshaft keyway good...key was in place and in new condition.

    [​IMG]

    Two pics of bearing. Some scoring going around, and I assume that little detent in the first one is normal (at about 10:30 in the pic)?

    [​IMG]

    [​IMG]
    #10
  11. DiabloADV

    DiabloADV Semi-Occasional

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    Aren't you guys hosting a tech day in two weekends? Maybe I should bring this up there...don't know if I can have it ready to reassemble by then.
    #11
  12. Boxer Metal

    Boxer Metal Mad Scientist

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    You could bring it up. I see 2 things looking ta the bearing. 1. pitting from old oil. Oil is actually acidic. 2. The bearing also looks like there might have been something in the oil. I know my good friend Bob is lurking in the back ground here. Lets wait for his post.:D
    #12
  13. DiabloADV

    DiabloADV Semi-Occasional

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    Also...I haven't had a look at the front bearing yet. I haven't got a proper tool to pull the sprocket off. I'm taking the crank down to the dealer this afternoon and the guy there will pull it for me.
    #13
  14. DiabloADV

    DiabloADV Semi-Occasional

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    What about the oil holes not being at 12 and 6? Doesn't that say something...?
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  15. robtg

    robtg Been here awhile

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    You nailed it Chris. That bearing looks like a later replacement. The originals were different. The red color code paint is too fresh for a motor that old.
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  16. DiabloADV

    DiabloADV Semi-Occasional

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    Replacement is in order, I assume? Or is this bearing serviceable?
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  17. Boxer Metal

    Boxer Metal Mad Scientist

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    I would question if it has spun in the engine case

    At this point I would pull the crank and really look at everything carefully. It kind of looks like someone has been in there before and maybe made a mess of things. Bob who posted after me has built all of my engines in the past and now that he has retired I have Ozzie up here in Chico do all of my bottom end work. I have to choose my battles here when it comes to time.
    #17
  18. robtg

    robtg Been here awhile

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    I'm not retired, just limited my work to mostly machine work.:D
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  19. DiabloADV

    DiabloADV Semi-Occasional

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    Motor is totally apart (but for the front main bearing is still on the crank). Everything else checks out and looks clean. My *hunch* is that the bike was well-serviced and taken care of. There are notes in the shop manual that came with it about a full top-end rebuild in 1974. But...I would bet the bike was hardly ridden after that. I got it from the second owner, who abused the crap out of it on fire roads for about six months. There was sand in the oil pan.
    #19