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Gearbox top mounting bolt

Submitted by adam_lynam on Sunday, 22 December, 2024 - 19:26
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Commando

Hello, wondered if anyone can advise on this please. The top gearbox mounting bolt on my 1971 750 only engages to about a third of the depth of the locknut. I bought a longer one from the NOC, it's a bit longer and has a hexagonal head instead of the self locking type but the thread pitch is different, does anyone know what type was used?
Please see the photos for the dimensions including the measurement across the cradle plate outer faces in case something else is amiss. 

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Submitted by paul_griffiths on Sunday, 22 December, 2024 - 20:45

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My Commando 1972 vintage has the cup square variety of top bolt such as you show with a length of 123.63mm from under the head to end of threads.

The nut is not shouldered such as you show in your picture but plain without the reduced section as per the parts book.
Oddly my parts list for 750s such as mine shows the "ordinary" looking bolt such as that which you show above but in fact it has the cup square type to add to your confusion!!
I would go for the cup square bolt simply because it takes out another threaded fastener you don't have to raum about using a second spanner across the frame.
Cradle across measures 96mm at its extremes.

 

Thanks for your time and help Paul,  Stan has pointed out there is a special nut used with the longer hexagonal headed bolt I've shown and it goes in from the right side of the bike. 

Submitted by stan_thomas on Sunday, 22 December, 2024 - 21:38

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It's 9/16 UNF. It does not use a locknut. The bolt screws in from the right into a special round nut that has tangs to engage with the slots in the engine plates (#35 here: https://andover-norton.co.uk/en/shop-drawing/210/gearbox-shell-gear-cluster-mounting-bolts-adjuster-countershaft-sprockets )

The correct bolt is 5" long. Engine plates are 3 3/4" o-o. (all -ish :) )

1970 had the coach bolt you describe. A Whitworth size apparently, see https://andover-norton.co.uk/en/shop-drawing/5/gearbox-shell-gear-cluster-mounting-bolts-adjuster-countershaft-sprockets

Guess you need a longer one.

Thanks again Stan, the special nut will be the missing link, that and the hexagonal headed bolt goes in from the right which would have saved me removing the primary drive, clutch and case to get the existing incorrect coach bolt out. Still it might have been a blessing, as I discovered the clutch has 3 steel plates and 5 fibre, 2 fibre against each other and also a 530 chain and rear sprocket with a new incorrect 520 gearbox sprocket. One thing leads to another. 

Submitted by paul_griffiths on Monday, 23 December, 2024 - 13:48

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I would be interested to know the thickness of the two back to back fibre plates as compared to one that is sandwiched between two steel plates purely as an indicator for wear.
The set up that you have found is not something I would even contemplate!

Submitted by tony_ripley on Monday, 23 December, 2024 - 13:56

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There is no problem with fibre to fibre as long as they are both going round together.
presumably it is just one way to get the stack height right.

Submitted by tony_gill on Tuesday, 24 December, 2024 - 10:31

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STACK HEIGHT
To derive the full clutch height you add all the seperate components including the FULL height of the pressure plate.
This is the FULL dimension filled by STACK HEIGHT of the components, between the backing plate in the clutch basket out to the diaphragm. From an engineering stand point this is the correct number.
Early clutch, original  stack height=1.167”
Late clutch, original stack heigth=1.172"
 

 



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