venting an airhead gear box

Discussion in 'Airheads' started by nella, Aug 24, 2010.

  1. nella

    nella Been here awhile

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    403
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    Raleigh, NC
    A while back I made a plug to go in the hole left when I removed the speedometer cable:

    [​IMG]

    I read that HPN offers a vented breather plug that has a tube that you attach to the frame.

    My question is should I make a breather arrangement for my plug? If so, why does the gear box need to be vented?

    Thanks,

    Scott
    #1
  2. Airhead Wrangler

    Airhead Wrangler Long timer

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    You already have a breather. The bolt holding your plug in has a hole down the length of it. That's the stock breather. Or you could replace it with a solid bolt and switch your plug to one of these:

    [​IMG]
    #2
  3. rkatapt

    rkatapt Indo-Kraut

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    I stuck a cork in my old speedo cable hole...
    #3
  4. east high

    east high Been here awhile

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    /thread
    #4
  5. Hawk Medicine

    Hawk Medicine Coyote's Brother

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    I have one of those vented plugs floating around the shop somewheres...

    As I understand it, the guys racing airheads need to vent the gear box because under stress, they get hot and can puke oil. Therefore, the simple solution is to vent the tranny out to the the same catch bottle that services the engine.
    #5
  6. nella

    nella Been here awhile

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    My speedo cable bolt does not have a hole in it (it's not the original bolt), so I guess I'll make up a new vented plug. Thanks all for the advice.

    Scott
    #6
  7. pommie john

    pommie john Long timer

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    Yes you need a breather. The 'box contains air and oil that expand as it is heated causing an increase in pressure in the 'box. The temp of the gearbox can vary quite a lot from sub zero to ( guessing here) 80 C.
    Without a breather to vent this pressure, it will be trying to get out of the seals and either finding its way into the clutch, or the shaft, or down onto the road.
    #7
  8. bikecat

    bikecat Long timer

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    What's the part number for the original vented plug for a R100Gs?

    Thanks
    #8
  9. rkatapt

    rkatapt Indo-Kraut

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    Bumping this thread up...

    Does anyone know where to get this breather stateside? I am wanting something a little more refined than the cork I've been using for a while. If I order from bmwboxersupplies.com it's €25,60 just to ship it to Dallas and the part itself is only €16,39.
    #9
  10. bpeckm

    bpeckm Grin!

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    Touratech supplies a "proper" cap, but uses the tranny vent bolt for venting... ($16.40 and in stock) .....If you are looking specifically for something with which to vent farther up (water crossings, anyone?), it looks like you might be able to drill and tap this piece for an air tube, and use a solid bolt rather than the stock vent-bolt....

    [​IMG]



    :D
    #10
  11. Jon-Lars

    Jon-Lars Jon-Lars

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    When you installed the replacement plug, did you remove the speedo drive gear from the tranny?
    It comes out through the plug hole.
    #11
  12. bpeckm

    bpeckm Grin!

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    Hmmm... if one were to pull out the drive cable, and to leave that drive gear in there, what might happen? I would think that is supported and kept in place with more than just the cable end?


    :ear
    :ear
    :ear
    #12
  13. nella

    nella Been here awhile

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    I think most folks just pull the cable and replace with some plug without fooling around with the drive gear inside.
    #13
  14. Rucksta

    Rucksta SS Blowhard

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    I cut an old speedo cable,pulled the guts out and ran it up under the tank to make a piped vent.

    Looks stock cost nothing.
    #14
  15. Jon-Lars

    Jon-Lars Jon-Lars

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    If anyone has a transmission on the shelf, remove the vented bolt and tilt the tranny over so that the drive gear slips out. It isn't retained in there by much. I think it has a boss on the bottom end that locates it in position inside the case.
    If you remove the speedo cable, the top end of the gear is no longer held in place against axial and longitudinal loads. The gear is still driven but is able to move out of its engagement zone. I don't like that situation.
    What is the drawback if you remove it? Are other components that are lubricated in there by the splash of that gear?
    Wish I know more about the innards of a 5 speed BMW gearbox.
    #15
  16. Box'a'bits

    Box'a'bits In need of repair

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    I've done exactly that. I also zip tied a bit of filter foam over the end of the cable outer to keep the dust out. I also siliconed the cable end where the boot goes, so that that area doesn't allow in water. And I have a solid allen headed bolt into the tranny.

    I think that the speedo drive is held in by the bolt into the tranny, not by the cable itself.

    Now I need to find a venting solution for the final drive that appeals to my 'airhead' sense, i.e. cheap & practical.

    My bike sees a few stream & river crossing, a bit of mud from time to time, & a lot of dust from gravel & dirt roads
    #16
  17. Jon-Lars

    Jon-Lars Jon-Lars

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    The bolt engages a plastic bushing on the end of the speedo cable. That bushing holds the end of the cable and, I think, helps locate the drive gear in position.
    #17
  18. Box'a'bits

    Box'a'bits In need of repair

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    Correct. :D
    #18