Catastrophic Belt Failure

marshg246

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Left or right?

I thought @marshg246 bought that stuff.
Not me - that's one of those like Clubman that I contacted but they went another way without even saying what they wanted for their stuff. Over the last several years I've bought out some private people but not companies.
 

Time Warp

.......back to the 70's.
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The adjuster could probably be a snail cam at the location of the inner spacer (03-0023) which would be in compression. (The stock spacer could have a larger diameter for a bigger footprint)

The top bolt which is a average fit in general could have a hardened shim washer under the head so any twisting force from tightening is minimised to the stock adjuster bush/body.
(That adjuster looks to have come away at the join of the thread into the body)

The loose fit of the bolt might also have an effect on things and the order the adjuster is adjusted, including resetting of the lock nuts after main bolts are tight.

JC would have just changed it and the muffler bearings on the side of the road.
 
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The adjuster could probably be a snail cam at the location of the inner spacer (03-0023) which would be in compression. (The stock spacer could have a larger diameter for a bigger footprint)

The top bolt which is a average fit in general could have a hardened shim washer under the head so any twisting force from tightening is minimised to the stock adjuster bush/body.
(That adjuster looks to have come away at the join of the thread into the body)

The loose fit of the bolt might also have an effect on things and the order the adjuster is adjusted, including resetting of the lock nuts after main bolts are tight.

JC would have just changed it and the muffler bearings on the side of the road.
I forgot to pack the muffler bearings.
 

Time Warp

.......back to the 70's.
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That would do it.
It looks to have two adjusters and one still in place on the primary side.

Maybe the 750 set up was fine with the captivated top bolt having a nut at the adjuster side to tighten things.
 
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https://shop.kingpincomponents.co.uk/norton/transmission/norton-primary-chain-adjuster-threaded-hole

I was planning on getting one of these but they were out of stock when I was putting it back together. RGM wants $26 per item for shipping. :rolleyes:

I hope that fits a Commando. With my luck it's for the older bikes.
I have used this with a Maney belt drive for many trouble free (I didn't say that,touch wood ) miles.
 

robs ss

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Well, maybe it's the powder coated engine plates - or something - but I have found that, despite properly torquing the top and bottom gearbox bolts, it has been wise to pull the primary cover off occasionally to check belt tension. Normally once a month (for me that's about 350 miles).
If it needs adjustment, normally about every third check, it is to loosen it a bit.
I'm not saying over tension contributed to your problem, just a topical observation from my bike.
 

grandpaul

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Also, some adjusters I have seen are not symmetrical, they have to be mounted the right way to line up properly with the square locating part, if fitted the wrong way the threaded rod will be under huge tension, which would be an obvious potential to cause it to snap.
That would be my guess.
 

Carl H

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As you can see the gearbox adjuster failed. I had planned on getting the second adjuster but wanted to try it with one first. Well that didn't work. Top bolt was tight, maybe not tight enough? The adjuster is original and a little rusty.

We had a good ride yesterday to the club meeting, but I didn't check the belt after the ride. I had checked it after the previous ride and even hot there was the correct play.

A call to AAA and the bike was picked up and safely home. I have it on the morgue table now and will get in there to clean it up.

And no, I'm not going back to a chain.
Left or right?

I thought @marshg246 bought that stuff.
Was the trans cradle power coated? I struck out in Daytona , because the race bike, Jerry Wood #3 was riding had been power coated and the trans bolts loosen up. . He was doing real good and running fast. Luckily he sensed something wrong and nothing got damaged much, and pulled out, Except my pride. No more power coat for me and lots of retorks to be sure.
 
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Was the trans cradle power coated? I struck out in Daytona , because the race bike, Jerry Wood #3 was riding had been power coated and the trans bolts loosen up. . He was doing real good and running fast. Luckily he sensed something wrong and nothing got damaged much, and pulled out, Except my pride. No more power coat for me and lots of retorks to be sure.
Original paint.
 
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Autopsy done. Stator and belt are terminal. I might upgrade to a 3 phase although I don't really need one. I need to check with @marshg246 to see what he has in stock.

The front pulley guide was munched. I emailed Steve Maney in case he has a front pulley still. Maybe I'll need to have one made. No idea how he attached them to the pulley.

I think I got away as I suppose it could have been worse. If it had been a chain let go it would have been uglier.

I should just go with the gearbox adjuster from @Holmeslice or RGM.
 

Fast Eddie

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Wow.

Swoosh, the autopsy ain‘t done yet matey... an autopsy is intended to find the CAUSE of death... not just expose symptoms...!

Here‘s my hypothesis:

The belt has clearly been forced outwards under a fairly strong and consistant pressure.

Why? That can only come from the clutch drum not being in line, being on an angle will make the belt track one way or the other. Yours has been angled such to force the belt outwards.

Why? If the chain has pulled the gearbox backwards, it will reach a point where the belt is stopping it being pulled further. At that point, because the sprocket is inboard of the clutch drum, it will start to pull it off centre in the direction that forces the belt out.

Why? For all this to happen in such low mileage Swoosh, for whatever reason, fairly conclusively points to a lose top gearbox fixing.

Why? I dunno. Not tightened correctly? Power coat crushing? One possibility is the lollipop was fitted the wrong way around, with the threaded rod miss aligned, this could have potentially prevented the top bolt from applying correct clamping forces even though it felt ‘tight’. Or...?

One thing I will say is that the fact that belt didn’t snap is a huge testament to that belt. Is it the original belt from Steve Swoosh, or have you replaced it from another source?
 
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As Ludwig says, be an idea to check for any wear on the mainshaft where the sleeve gear bushes are.. Mine had significant wear on a 20k miles only bike......
As a Mk3 owner I'm not overly familiar with the adjuster bolt set up, but looks to be rust residue internally???
 
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Wow.

Swoosh, the autopsy ain‘t done yet matey... an autopsy is intended to find the CAUSE of death... not just expose symptoms...!

Here‘s my hypothesis:

The belt has clearly been forced outwards under a fairly strong and consistant pressure.

Why? That can only come from the clutch drum not being in line, being on an angle will make the belt track one way or the other. Yours has been angled such to force the belt outwards.

Why? If the chain has pulled the gearbox backwards, it will reach a point where the belt is stopping it being pulled further. At that point, because the sprocket is inboard of the clutch drum, it will start to pull it off centre in the direction that forces the belt out.

Why? For all this to happen in such low mileage Swoosh, for whatever reason, fairly conclusively points to a lose top gearbox fixing.

Why? I dunno. Not tightened correctly? Power coat crushing? One possibility is the lollipop was fitted the wrong way around, with the threaded rod miss aligned, this could have potentially prevented the top bolt from applying correct clamping forces even though it felt ‘tight’. Or...?

One thing I will say is that the fact that belt didn’t snap is a huge testament to that belt. Is it the original belt from Steve Swoosh, or have you replaced it from another source?

More likely is that when I tightened down the top bolt the weakened adjuster failed. When that fails even a tight top bolt can't hold it in place.

The solution is dual adjusters for redundancy (also to replace 50 yr old lollipops).

That is a replacement belt. The original belt is still in great shape and I'll use it until I can find a replacement.
 

maylar

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Yes, check the mainshaft for runout. I had catastrophic failure of the chain primary (grenaded alternator rotor) that bent my mainshaft. Back in those days Old Britts had used parts for sale, Fred found me a shaft with .003" runout.
 
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