Gearbox Mainshaft Nut. Can't tighten. 1975 MKIII

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DesertRat

DesertRat

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Dec 5, 2010
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Turning a small problem into a large one here.

Removed the outer & inner gearbox covers to replace a few gaskets and seals. No problem disassembling etc.. On re-asssembling I can't get the mainshaft nut (behind the clutch operating body lock ring) tight. The shaft turns with 10-15 lbs of torque applied.

It's in top gear and although the manual says have someone hold the rear wheel to prvent turning, that's not a problem.

I'm not familiar enough with the system to understand what is allowing the whole shaft to turn while trying to tighten the nut. The manual diagrams are not helping me.

What am I missing here?

Thanks for any thoughts

Mike

PS Since it's my 1st post here (I'm a little more regular on Brit Bike) I should say hi to all & thanks for the help.
 
Welcome to Commando trouble shooting central.

I've stuck a hammer handling or stick or long wrench in the spokes to lock up to nip up. I think you are describing trouble getting the cable lever gizmo to snug down in line and then lock ring torqued down. It takes me a bit trial-error to get them both nipped up and inline. Fortunately this don't tend to unscrew much so don't need to be misserable tight. I use a drift to bump lock ring pretty snug, hoping this happens so it leaves it aimed well for cable run. Hope that's covers it.
 
Can you put a block of wood in the drive sprocket/chain to keep it from turning while it is in gear instead of using the brake? Is the brake slipping?

Dave
69S
 
If the gearbox is in top, then the mainshaft should be locked directly to the sleeve gear and drive sprocket (by mainshaft 3rd.) so if the mainshaft is turning then the sleeve gear and drive sprocket should turn by the same amount?

If the brake is holding the wheel, are you sure the mainshaft movement isn't being caused by the MkIII cush drive compressing as you apply torque to the nut because any amount of rear wheel sprocket movement will be magnified at the mainshaft due to the difference in sprocket sizes.
 
The rear wheel is not even trying to turn.
Couldn't lock the rear brake in any case as the footrest/master cyl is hanging in space as I fiddle with the gearbox.

But, the chain/rear wheel does not seem to be an issue. It's not trying to rotate as I tighten the nut
 
Are you sure you are in 4th gear, try another gear. It should lock up, even if you have to lock the sprocket somehow.

Dave
69S
 
The shift fork is at the top of the window, should be 4th gear.
But I have moved it to other positions with the same result.
I don't think the final drive is even trying to rotate.
 
DesertRat said:
The rear wheel is not even trying to turn.
Couldn't lock the rear brake in any case as the footrest/master cyl is hanging in space as I fiddle with the gearbox.

But, the chain/rear wheel does not seem to be an issue. It's not trying to rotate as I tighten the nut
Click to expand...

That's odd, as the rear brake needs to be held full-on to prevent the mainshaft/sleeve/sprocket from turning?
 
I was wondering too if it's the clutch slipping.

Dave
69S
 
Thought the same thing. I was trying to avoid taking the primary apart.

As I said. Small job into big. Next time I'll just throw another rag on the oil drip.

Thanks, I'll report back
 
DesertRat said:
Thought the same thing. I was trying to avoid taking the primary apart.
Click to expand...

The clutch will probably start to slip as you attempt to tighten the mainshaft nut-with the box in 4th gear, you need lock the mainshaft by applying the rear brake (or find some other means to prevent the rear wheel from turning) as you apply torque to the nut.
 
Something does not compute, Could you give a part number on which nut is giving the trouble. If you can kick start the thing then clutch is holding good enough.
 
#12 in group 7 is the bolt that clamps shift lever on. My look closer did find parts book showing shift lever installed with bend and peg reversed from reality. I"m putting a Combat together, so soon facing same area, so hoping to get caught up on pointers too, but this post is a mystery, a nut don't nip up but which 'nut' and can't hold gearbox shafts from turning, how can that be w/o obvious reason?
Hey it could happen to me too, so waiting for happy ending.
 
hobot said:
#12 in group 7 is the bolt that clamps shift lever on. My look closer did find parts book showing shift lever installed with bend and peg reversed from reality. I"m putting a Combat together, so soon facing same area, so hoping to get caught up on pointers too, but this post is a mystery, a nut don't nip up but which 'nut' and can't hold gearbox shafts from turning, how can that be w/o obvious reason?
Hey it could happen to me too, so waiting for happy ending.
Click to expand...
Come on Hobot. You need to cut that white lightning with a little more schnapps. 75 MKIII! Just click on the link.
 
OK

Problem identified

Nut #12 is stripped. Not exactly sure how or when it happened, but it explains the no tighten/no turn the rear wheel mystery.

I didn't see it before, because I couldn't get it off either.

And, my clutch looks great. So I guess that's a partially happy ending.

Looks like Fred has some in stock, so for today , on to the next project !

Thanks for all the thoughts, all were corrrect, "It didn't make sense"
 
Thanks for letting us know. I hate those threads that leave everyone hanging. It's like those news stories you never hear the end of.

Dave
69S
 
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