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Gearbox breather #757639
11/30/18 4:19 pm
11/30/18 4:19 pm
Joined: Aug 2009
Posts: 1,009
gastonia .. NC
L
limeyrider Offline OP
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limeyrider  Offline OP
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Joined: Aug 2009
Posts: 1,009
gastonia .. NC
Doing some work on the gearbox of my 1966 , 650 SS, I noticed that some of the internal nuts had a coating of rust, further investigation shows that there is no breather on the gearbox .... I think that the later boxes had a vent tube alongside the clutch cable attachment area ... the early boxes had a hole in the oval inspection cover ?, the previous owner has replaced the stock oval inspection cover with a INOC logo badged plate ... no breather hole.
What would be the best route to take .... drill the top of the outer cover ?, drill the inspection plate ?.... idea's ?.

Thanks.

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Re: Gearbox breather [Re: limeyrider] #757651
11/30/18 6:38 pm
11/30/18 6:38 pm
Joined: Nov 2006
Posts: 523
Wisconsin, USA
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franko Offline

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franko  Offline

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Joined: Nov 2006
Posts: 523
Wisconsin, USA
For the amount of air moved and at the slow pace it would move, would the clutch cable be able to handle it?

Re: Gearbox breather [Re: limeyrider] #757660
11/30/18 7:47 pm
11/30/18 7:47 pm
Joined: Dec 2004
Posts: 9,667
Scotland
kommando Online content
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kommando  Online Content
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Joined: Dec 2004
Posts: 9,667
Scotland
Well the later breather does not stop the condensation.

Re: Gearbox breather [Re: franko] #757732
12/01/18 3:36 pm
12/01/18 3:36 pm
Joined: Aug 2009
Posts: 1,009
gastonia .. NC
L
limeyrider Offline OP
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limeyrider  Offline OP
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Joined: Aug 2009
Posts: 1,009
gastonia .. NC
I don't know Frank, I guess that there is not a whole of air movement within the casing, Mick Hemmings makes a point in his DVD to make sure that the vent holes are clear ??

Re: Gearbox breather [Re: limeyrider] #757744
12/01/18 6:18 pm
12/01/18 6:18 pm
Joined: Nov 2003
Posts: 755
Orlando fla/ shady valley Tn
B
bill50cal Offline
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bill50cal  Offline
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Joined: Nov 2003
Posts: 755
Orlando fla/ shady valley Tn
dyno dave did a bunch of testing and could not see any pressure after plugging all the openings. and from what i have seen there is no need for a breather. you will not stop the humidity in it and rust as it will happen just from the temperature changes and can vary from location to location because of the humidity where you are at.


windy
72 combat
switchbackcreek.com
Re: Gearbox breather [Re: limeyrider] #757761
12/01/18 9:25 pm
12/01/18 9:25 pm
Joined: Aug 2001
Posts: 2,361
Oztralia
R
Rohan Offline
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Rohan  Offline
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Joined: Aug 2001
Posts: 2,361
Oztralia
Competition machinery usually has breather tubes (up high) so that if the gearbox is suddenly chilled,
it doesn't suddenly suck in a mouthful of water though any available entrance points.
Thats the only time you'd likely see any real pressure differences, apart from the box warming up. ?

Nortons gearbox design has a heap of steel bits that never get any oil lube splashed on them.
Nortons breather addition was a half-baked solution to the wrong problem ?
I brush a trace of grease over them on assembly - and its usually still there on disassembly years later.

AMC/Burman boxes from years earlier had a grease nipple for lubing the mechanism, sometimes thungs go backwards in design ??

Re: Gearbox breather [Re: Rohan] #757778
12/01/18 11:53 pm
12/01/18 11:53 pm
Joined: Aug 2009
Posts: 1,009
gastonia .. NC
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limeyrider Offline OP
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limeyrider  Offline OP
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Joined: Aug 2009
Posts: 1,009
gastonia .. NC
Thanks to all for the comments, as the rusting on the internal nuts is very light, and non existent on the gears/ bearings etc. .... I will leave well alone..... no holes !.
Thanks again.

Re: Gearbox breather [Re: limeyrider] #758429
12/07/18 1:45 pm
12/07/18 1:45 pm
Joined: Aug 2001
Posts: 1,290
Hamilton, Mass. USA
D
Dave Comeau Offline

Crew Chief
Dave Comeau  Offline

Crew Chief
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Joined: Aug 2001
Posts: 1,290
Hamilton, Mass. USA
from my web pages:
http://atlanticgreen.com/gearboxpressure.rtf

Spring 2012

Gearbox pressure myth

Any gearbox breather discussion is a WAY over sold mod, even to the point of being ridiculous and should not be a real concern. Internet articles and forum postings on this subject are in my opinion of very low credibility. This is why.

I built a pressure test rig using a very low-pressure high-resolution gauge, regulator and ball valve I did a series of pressure tests of the AMC gearbox on my 72 combat. While the test included the demonstration of my clutch rod seals ability to restrain oil leakage, I was actually trying to debunk what I believed to be an unsubstantiated old wives tail about gearbox breathing/venting deficiency and the need for further breathing mods. This test was demonstrated at a few of the national rally tech sessions, and remained installed for an extended period of time to be able to redemonstrate the test at local tech sessions.

The first main leakage path is the path in the main shaft... this is what my CCRS fixes.
A close second leakage path would be the normal early vent in the little round access cover or the 850&MKIII brass tube style vent.
The third path is the clutch cable entrance. This can NOT hold pressure, even if you seal around the cable with RTV, the leakage also goes up the cable itself up to the handle bar.
A minor but variable fourth leakage path is the between the main shaft and sleeve gear bush.

In the final test the cable exit was totally plugged with a small rubber stopper and tested with soapy water for no leakage. Even with all the passages sealed, try as I might to actively pressurize the gearbox after totally sealing leakage paths one, two and three, there was no ability to view any residual pressure once the ball valve was shut to stop the regulated compressed air source. All remaining air pressure vented through the sleeve gear. The very high resolution...low pressure gauge went to zero quite quickly, soapy water bubbles showed the air leaving. The results were as I expected, and the normal gearbox is absolutely NOT able to hold pressure, especially once the clutch cable is reinstalled.

I would have no problem using an undrilled MKIII access cover or without the MKIII brass vent on AMC gearboxes. The results of the test show that any theoretical air expansion causing a pressurization of the gearbox due to heat from operation is in my opinion totally bogus and without any merit what so ever.


dynodave
BSA 3 1961-1963
Ducati 3 1992-2002
Norton many 1951-1975
87 Serv-Equip 100HP MC brake dynamometer,
Re: Gearbox breather [Re: limeyrider] #758527
12/07/18 10:05 pm
12/07/18 10:05 pm
Joined: Aug 2001
Posts: 2,361
Oztralia
R
Rohan Offline
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Rohan  Offline
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R

Joined: Aug 2001
Posts: 2,361
Oztralia
Sealing around the clutch cable entry with rtv or tape etc is fairly helpful, this helps prevent the box sucking in water if you meet up with a rainshower anywhere.
That yellow look to the gearbox oil is usually from this......


Moderated by  Dave Comeau 


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