Holy crap a hole in my inner primary cover!

Larry S

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Here's the back story. My last ride started out fine but didn't end quite as well. I was rolling down the road at about 60 mph when suddenly there was a thunk and a shudder from the bike followed by significant vibration. I pulled of the road to find oil all over my back wheel. Upon closer examination I found a hole in the lower rear of my inner primary cover. Today I finally got my bike to it's new heated winter shed and removed the outer primary cover. The first thing that I saw was a washer laying in the bottom. I then noticed that the nyloc nut that secures the inner primary to the chaincase steady stud was missing. That nut rattled around in the chaincase until it got caught by the clutch chainwheel causing it to break out the inner case. (The thunk and shudder) I also found some small metal bits in the cover that I think came from the starter sprag. The starter still worked correctly but when I rebuilt primary the sprag looked to be in bad shape. Other than replacing the inner primary cover, the nut and starter sprag everything else looks good to me.
 

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Fast Eddie

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Oh dear, doesn’t look nice, but you're probably lucky it wasn’t worse.

If the nut got caught / jammed by the clutch sprocket hat hard you need to check your main shaft and even crank shaft to ensure they’re true. Most likely victim would be the gearbox mainshaft I’d imagine.
 
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cliffa

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As FE said, it could have been a lot worse.

To me the amount of protruding stud looks short. With a washer on it does the nylon section on the nut actually reach the threads ? Just wondering if that’s why it fell off.
 

robs ss

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The other possibility - nylocs don't last forever.
Critical ones should be assessed for "grip" and replaced every 6 or so "tightenings" (new word!)
 

L.A.B.

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As FE said, it could have been a lot worse.

To me the amount of protruding stud looks short. With a washer on it does the nylon section on the nut actually reach the threads ? Just wondering if that’s why it fell off.

Yes, possibly the support stud '23' is fitted incorrectly due to the parts diagram error that shows the stud facing in the wrong direction...


...as the long threaded end of the stud should be to the outside as drawn in the Mk3 manual Fig. C45, (inset), below. The thin nut '25' should be on the outer end to support the case and the outer nut to tighten against...


...and can require additional packing washers between the hexagonal section and the cradle for there to be sufficient thread for the locknut.
My Mk3's locknut was the split metal type so possibly what the fragments of metal in the last pic could be.


Edit: The strip-down pictures in the rebuild thread shows the stud was fitted correctly at the time and had the extra washers.

Edit 2:
The pic below I think shows the stud is fitted correctly but the case has moved away from the inner thin nut and washer by about 3/16" so would account for the stud appearing to be too short.

 
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Deets55

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Take a good look at your primary chain also. When my clutch adjuster nut came off it damaged one of the side plates. Thankfully that was the only damage in my case.
Pete
 

Larry S

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Oh dear, doesn’t look nice, but you're probably lucky it wasn’t worse.

If the nut got caught / jammed by the clutch sprocket hat hard you need to check your main shaft and even crank shaft to ensure they’re true. Most likely victim would be the gearbox mainshaft I’d imagine.
I plan on using a dial indicator to check both shafts.
 

Fast Eddie

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Does this mean that I didn’t have the thin nut adjusted against the chaincase correctly?
No, flipping it over won’t reduce the length in that respect. But it puts only a short portion of thread into the chaincase, which is most likely too short for the locking part of the lock nut to engage on. Hence the failure.
 

L.A.B.

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Does this mean that I didn’t have the thin nut adjusted against the chaincase correctly?

Only you would know.

Whether it occurred before or after the calamity, an enhanced view of your picture, below, shows the inner nut some distance away from the chaincase...


...Without the inner nut (and washer) up against the chaincase then the outer nut would not have tightened fully (and possibly wasn't screwed on far enough for the locking part of the nut to be in contact with the stud thread?), consequently, it unscrewed and fell off.

No, flipping it over won’t reduce the length in that respect. But it puts only a short portion of thread into the chaincase, which is most likely too short for the locking part of the lock nut to engage on. Hence the failure.

After looking at the enhanced pic I'd say the stud does look correct. It's the nut position that's not as it should be.
Unlike pre-Mk3 that uses shims to adjust chaincase alignment, the Mk3 alignment is achieved by adjusting the positions of the inner and outer nuts and then locking them up (Edit: The inner nut is still accessible from below with the chaincase in position). My apologies if you already knew that.
 
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Tornado

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Your last pic in original posting shows chips that look very much like those that my original chain shed off its rollers when one of the three inner primary case bolts came loose and went for a rattle. Have a careful inspection of all chain rollers.
 

robs ss

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I’m going to try and get it welded up if I can find someone. It will never be seen.
I had a Commando inner primary welded, to suit fitment to my '62 500 single.
These items are quite porous.
My advice would be:
1. Make sure the wife is nowhere near
2. Bake the case in the oven at 150 Celsius
3. Remove and wipe the oil residue off with thinners
4. Repeat steps 2 and 3 something like six times until no significant oil is evident.
Your welder will thank you and you'll have a better weld repair.
Cheers
 

Larry S

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The autopsy is now complete. The fixing stud was loose at the cradle and at the inner chaincase resulting in the nylox nut working loose, rattling around in the chaincase until it blew a hole in the inner cover. Found the nut wedged behind the chain tensioner. I'm going to try and get the inner cover welded up and put her back together with much greater care.
 

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I had a Commando inner primary welded, to suit fitment to my '62 500 single.
These items are quite porous.
My advice would be:
1. Make sure the wife is nowhere near
2. Bake the case in the oven at 150 Celsius
3. Remove and wipe the oil residue off with thinners
4. Repeat steps 2 and 3 something like six times until no significant oil is evident.
Your welder will thank you and you'll have a better weld repair.
Cheers

And please avoid oven cooked food for a while
 
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