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Old 08-24-2010, 10:23 AM   #1
nella OP
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venting an airhead gear box

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



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
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Old 08-24-2010, 10:46 AM   #2
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Quote:
Originally Posted by nella
My question is should I make a breather arrangement for my plug? If so, why does the gear box need to be vented?
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:

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Old 08-24-2010, 11:02 AM   #3
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I stuck a cork in my old speedo cable hole...
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Old 08-24-2010, 11:38 AM   #4
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Quote:
Originally Posted by rkatapt
I stuck a cork in my old speedo cable hole...
/thread
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Old 08-24-2010, 12:23 PM   #5
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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.
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Old 08-24-2010, 02:16 PM   #6
nella OP
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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
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Old 08-24-2010, 05:03 PM   #7
pommie john
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Quote:
Originally Posted by nella
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

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.
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Old 08-24-2010, 05:35 PM   #8
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What's the part number for the original vented plug for a R100Gs?

Thanks
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Old 03-18-2011, 10:25 AM   #9
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Quote:
Originally Posted by Airhead Wrangler View Post
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:

snip
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.
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Old 03-18-2011, 10:37 AM   #10
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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....





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Old 03-18-2011, 12:31 PM   #11
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.
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Old 03-18-2011, 01:56 PM   #12
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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?




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Old 03-18-2011, 05:34 PM   #13
nella OP
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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.
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Old 03-18-2011, 05:58 PM   #14
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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.
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Old 03-18-2011, 06:05 PM   #15
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.
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