need dimensional data - gearbox outer cover

Time Warp said:

With a hammer given the no thread for 1/4" on the tube.

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1/4"? Perhaps you do need glasses?



Anyhow, regardless, I suggest Joe doesn't attempt to remove the breather unnecessarily as the 'ears' of the screwdriver slot break off easily which could mean shortening the tube and recutting the slot to unscrew it.

L.A.B. said:

1/4"? Perhaps you do need glasses?

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Last time I looked a thread is applied down the shank not across it, that being as I stated, from the top of the tube to the start of the thread is around a 1/4 inch, the diameter is not relevant.

The item in question is obviously not the stock part due to the major difference in height for one.

I will add the pictures again for posterity.

My Norton.
I would like to see you screw that in any further.



The Joe bike.

Time Warp said:

Last time I looked a thread is applied down the shank not across it, that being as I stated, from the top of the tube to the start of the thread is around a 1/4 inch, the diameter is not relevant.

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I see what you mean, now.

Time Warp said:

The item in question is obviously not the stock part due to the major difference in height for one.

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Maybe not? And 'tall' or 'short' it still looks like the typical breather and not some other part.

I think there may be some confusion here - L.A.B.'s pic showing .186 diameter (mine also specs at .186). that corresponds with a tech, how-to article on Old Britts home page on the upgrade. in the article, it says to drill and tap for a 10-32 vent fitting (.186). i'm thinking TimeWarp is indicating there is approximately 1/4" of the vent above the case, where as mine is near flush. the casting thickness would not allow for a counterbore, so it has to be a full thread vs. partial thread thing. that would indicate two different parts. here is an internal pic of my gearbox...



from Old Britts - Eliminating oil leaks due to pressure build up in the Commando gearbox
by Fred Eaton



it would be interesting to see pics of other factory installed vents on Mk2/Mk2a commandos. as I said, it appears that my engine/transmission have never been separated from the frame, and if this was a mod, I can't see how a previous owner drilled and tapped without removing the gearbox.

joe czech said:

the casting thickness would not allow for a counterbore, so it has to be a full thread vs. partial thread thing. that would indicate two different parts. here is an internal pic of my gearbox...

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It seems the right length fror the breather but the only way to find out would be to unscrew it, but if tight, it's probably best not to try.

joe czech said:

from Old Britts - Eliminating oil leaks due to pressure build up in the Commando gearbox
by Fred Eaton

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This has been discussed before and it's not possible for pressure to build up in the gearbox due to the cluch cable entry etc.

joe czech said:

it would be interesting to see pics of other factory installed vents on Mk2/Mk2a commandos -

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Here's another (Eng. 317975):
https://www.accessnorton.com/Norton...nsmission-rebuilding.11179/page-2#post-154797

joe czech said:

I think there may be some confusion here - L.A.B.'s pic showing .186 diameter (mine also specs at .186). that corresponds with a tech, how-to article on Old Britts home page on the upgrade. in the article, it says to drill and tap for a 10-32 vent fitting (.186). i'm thinking TimeWarp is indicating there is approximately 1/4" of the vent above the case, where as mine is near flush. the casting thickness would not allow for a counterbore, so it has to be a full thread vs. partial thread thing. that would indicate two different parts. here is an internal pic of my gearbox...

Click to expand...

Well that is an oddity, I did expect to see little inside the case, assuming wrong.


Yes, I measured the vent on my Dec73 Mk2a at around a 1/4" top of the tube to the inner cover surface, the overall length is 3/4"

Maybe there was two versions as the one you posted by all accounts looks like as per but more thread allowing it to be seated lower.

L.A.B. said:

This has been discussed before and it's not possible for pressure to build up in the gearbox due to the cluch cable entry etc.

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My own thinking has been along those lines and I may have actually stated that on the previous page but I wonder if we should challenge that thinking? The factory obviously thought the breathing could be better, there are common leaks down to the primary and out the shifter. We only posited that the gearbox oil magically found it's way into the primary and some have put a bandaid on the end to prevent it.

What if there is more pressure in the gearbox that we assume? I'm temped to put a big hole in the inspection cover and run a hose up and see what happens.

If it prevents the shifter leak, which is currently pronounced, then that might start to provide some evidence.

swooshdave said:

What if there is more pressure in the gearbox that we assume? I'm temped to put a big hole in the inspection cover and run a hose up and see what happens.

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IMHO the biggest contributer to the shifter leak is to much clearance in the shift shaft bush. it allows the shaft to move and with that movement it acts as an oil pump and forces oil through the seal. again IMHO even a new bush is to loose for a proper seal.

swooshdave said:

the shifter leak, which is currently pronounced, then that might start to provide some evidence.

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