Lighter oilhead wrist pins in an airhead

Discussion in 'Airheads' started by drjason, Apr 8, 2014.

  1. drjason

    drjason awesome

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    I googled this and searched the forum and I couldn't find anyone having discussed this yet, but it seems like it is worth considering:

    Success: Soaking piston in carb cleaner removes carbon very well.
    Failure: Soaking wrist pin (gudgeon pin) in carb cleaner removes chrome very well.
    Solution: Call re-psycle for a good used wrist pin. 11 25 1 335 474 GUDGEON PIN - 22mmOD X 13mmID X 74.31mmL

    What arrived happened to be what I figure is an oilhead wrist pin. 11 25 1 341 365 GUDGEON PIN - 22mmOD x 17.5mmID x 74.31mmL.

    It looks like it would fit just perfectly. The original wrist pin is 147grams, the oilhead wristpin is 83grams. Weight savings of 64g of reciprocating mass per piston.

    [​IMG][​IMG]


    Now I need to decide whether to send this back and order a proper airhead wrist pin, or keep it and ask for another for the other side.

    Question#1: Is there a heat/stress reason that the oilhead wrist pin shouldn't be used in an airhead?

    Question #2: Why hasn't this been discussed before (or is my Google-Fu just not up to snuff)?
    #1
  2. Cogswell

    Cogswell Spudly Adventurer

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    I think the airhead pistons are quite a bit heavier than the oilhead pistons, no ? There would be more stress on the thinner wrist pin.

    :ear

    Mike
    #2
  3. BOETJE

    BOETJE Been here awhile

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    #3
  4. Stagehand

    Stagehand Imperfectionist

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    seems like a good enough idea. :hmmm
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  5. bmwrench

    bmwrench Long timer

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    That looks to me to be an aftermarket lightweight pin for an Airhead. It's been a while since I had an Oilhead apart, but my memory is their pins are heavier than the one in the picture.

    As far as I know BMW wristpins are not chromed. Are you sure yours is unusable?
    #5
  6. chasbmw

    chasbmw Long timer

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    Carb cleaner may be aggressive, but I don't think that it can remove chrome.

    There was a craze for fitting lighter wrist pins some years ago and many claims were made about the results, however some of the wrist pins had a habit of breaking.

    Nowadays you can get the same result by fitting lightweight pistons, but without the breakages!

    I would suggest that you review what you have got
    #6
  7. 100RT

    100RT Long timer

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    Lightened pins were sold thru the Luftmeister brand. I also have heard of them breaking. I had a new set and got rid of them.
    #7
  8. bmwrench

    bmwrench Long timer

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    I never had a Lufmeister pin, which were made by Western Wristpin fail. I have heard-and only heard-that those sold by CC Products had a habit of breaking. These were made by a different manufacturer.
    #8
  9. Stagehand

    Stagehand Imperfectionist

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    But not actual BMW pins, right?
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  10. supershaft

    supershaft because I can

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    I have seen aftermarket lightweight wristpins break but I didn't know what brand they were. The stock ones are way too beefy. A lot of even stock pins nowadays are tapered on the inside for lighter weight and strength were its needed.

    Siebenrock claims their pistons are so much lighter but most all the weight savings is in the shorter wrist pins, not the pistons. Misleading AFAIAC.

    All wrist pins are chromed and then centerless ground down to the a couple of .0001". Like fork tubes but a lot more accurately ground.

    I still have stock pins in my bike. If I went for less weight, I would hone out some stock ones. Some of the aftermarket ones go too far in weight savings IMO.
    #10
  11. bajaburro

    bajaburro Ancient Adventurer

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    i've had them in my 91gs for many years.best and cheapest thing you can do to improve a gs imho.
    #11
  12. AntonLargiader

    AntonLargiader Long timer

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    Oilhead pistons are 59mm across the flats, while Airhead pistons are 67mm across the flats. That's not an Oilhead wrist pin.
    #12
  13. mike54

    mike54 You don't get me

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    Care to expound on that? I'm reading these posts and thinking I'll bet I couldn't tell the difference if my gs had lightened wrist pins or not. But maybe I could. :dunno
    #13
  14. BOETJE

    BOETJE Been here awhile

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    But the OP measures 74.3 mm for his air head pin....

    So you actually had oil head pins in your air head ?

    I'm confused now .....
    #14
  15. drjason

    drjason awesome

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    The thin wall pins are clearly dimensionally compatible with my R100 Pistons. So they may not be oil head pins. I guessed based on the photos of the individual part on the fiche. Although the fiche lists size for airhead pins, it does not for the oil head.

    I asked the shop to send me one more of each, and I'll decide on Sunday.

    As for the plating, the chrome is definitely gone in the middle where it was exposed. I even took the buffing wheel to it to see if it would get shiny again. But it actually is textured and is unusable.
    #15
  16. drjason

    drjason awesome

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    Well, I called Re-Psycle and asked them to send me one more of the light wrist pins and one stock. I was leaning towards using the light wristpin because it seemed good quality, but, unfortunately, the second light wrist pin is EVEN THINNER. Again, the outside dimensions are identical to the stock and it would work, but this one is only 74g.

    On the top are the first light pin, then the old one. On the bottom are the ultra-light pin, and then a good, used stock.
    [​IMG]

    So I guess I will go stock because I can't wait anymore, and the stock are the only two I have that weigh the same. But I would love to know who made these wrist pins and what their track record is.
    #16
  17. bmwrench

    bmwrench Long timer

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    It's impossible to know for sure, but they are probably both Luftmeister. Years ago, I spoke to the man who ran Western Wristpin. Against his better judgment, he had made some .080" wall pins for Luftmeister and was very skeptical about their likelihood of survival.
    #17
  18. chasbmw

    chasbmw Long timer

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    When you get the new original wristpins, use a digital scale to check that each piston and wristpin combo weighs the same. use a file on the end of the wrist pin of the heavier assembly to even them up.

    It's just a very quick and dirty way of removing a bit of imbalance on an airhead.
    #18
  19. drjason

    drjason awesome

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    The problem is finding a digital scale accurate enough to be useful. The one I have can only measure in increments of one gram, or a tenth of an ounce. Does anyone have suggestions on a reasonably priced scale with enough accuracy to be useful in this regard?
    #19
  20. disston

    disston ShadeTreeExpert

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    I have a Ohaus Triple Beam scale. Way accurate down to maybe 0.1 gram. Much more than needed. These are available used. Very common.

    The one I have used to be my X'es dealers scale. I've thought maybe I should somehow clean it but haven't bothered.

    [​IMG]
    #20