Wear pattern on rear sprockets

[mikeb]

I just renewed sprockets and chain on the A10 as the chain had a massive high spot – near 2”. When tightened (allowing for the high spot) it would still stetch quickly and lash about esp at low speed making for an unpleasant ride. I have to say chain and sprockets was not the funnest of jobs on the A10 and the first time I’ve used bearing retainer as the bearing slightly turned in the new sprocket. At least its not a plunger.
Anyway, the old sprockets show a strange wear pattern:

• The first pic below is the front/outside of both sprockets – it shows some scalloping on the rear sprocket teeth, not the gearbox sprocket.
• The second pic shows the back/inside of each – showing deep scalloping on the inside of the gearbox sprocket teeth, but not the rear sprocket
• The third pic shows the worst side of each. Note the scalloping on the rear sprocket is much worse on the left of the pic.

Taken together it suggest to me the chain was not running parallel, aka, that the rear sprocket was outside of the line from the front sprocket. Hence the chain was grinding out the inside of the gearbox sprocket and the outside of the rear sprocket.
So:

• is the gearbox out line? I did find the bottom pivot bolt had lost a nut so there may have been some movement there.
• Or is the rear sprocket sitting too wide? But it aligns with the chainguard and I believe the usually spacers and bits are in place. I checked in on some glass and the sprocket was flat.
• Or am I overthinking this again. The primary is back on so hard to examine much further atm.

All thoughts very welcome.
Thanks

[groily]

Fairly clear they have not been aligned, as you say, and that either the gearbox or the rear wheel, or both, have also been at an angle to one another, causing the unequal side-to-side wear on the rear sprocket. Quite surprised that the split link or a side plate hadn't failed on the chain actually, with that degree of chafing.

You're not overthinking, but I think that unless it's 'just' a rear wheel alignment thing, you might have moved too fast towards reassembly. Think I'd want to be able to get a long straight edge across the sprockets to be sure that all is good for your expensive new bits and I don't think you can do that easily with the primary side assembled again.

I suffered from a similar set of problems with another bike, and in the end had to beef up the primary chain tensioning system to ensure the the 'box stayed straight, and rebuilt the rear hub/axle to be certain it was 'right', with new sprocket, bearings etc and spacers checked for dimensions against other bikes & whatever data I could find. Boring - but with expensive new sprockets (and also the new bearings) at stake I think worth it. 
Haven't had chain trouble since, but I had actually broken  two good quality chains before. The first one I put down to bad luck  . . . but a second one - no way!

[mikeb]

Thanks for your comments groily. So it wasn’t raw power breaking chains on your a10? I recall someone dropping a chain on the road once on a japanese 4 when trying to wheelie off the lights in front of me. Not the effect he was hoping for.

Yes I think I put it back together too soon. I only examined the bits tidying up at the end of the job to decide if they were for the box or the bin. Optimistically, they were deep in chain grime so the wounds didn’t look new. That may be why the chain held up. Still… that’s wise counsel to have another look.

So I’ve removed the primary/clutch again and had a quick look with a piece of steel across the rear sprocket. At first glance, the rear sprocket looks about 1/8” inboard of the gearbox sprocket – the opposite of what that wear pattern would suggest. So that 1/8” may be wheel alignment or the shape of the piece of steel.

I’ll have a better look in the morning with fresh eyes

[groily]

So it wasn’t raw power breaking chains on your a10?

Ha! Not too much raw power in my (now departed) A I'm afraid. I did break the chain on it one time though, when it pîcked up a chunk of scrap metal on a country road.  Made a terrible mess and ripped several teeth straight off the rear sprocket into the bargain. But that was an exceptional one-off.
The problematic steed that snapped 2 in succession was actually a 650 AJS, for which I contrived a second gearbox adjuster cobbled up from very sturdy bits of any old iron and fitted on the opposite side to the standard one to keep things straight. (The standard affair is a somewhat hit and miss push-me pull-you sort of thing on some of the AMC 'boxes.) Some wiggle in the rear wheel department also didn't help. Fortunately resolved - touch wood - this past few thousand miles.
That beast  isn't a rocket ship either, but is quite quick for the era with a good 10 more horses than my A had. Because they also rev like hell (7000 limit), it gets thrashed through the gears a bit. That wouldn't have helped either as both breakages occurred on changing up into third at somewhere north of 50mph. The second one demolished one of the lugs on the crankcase that holds the inner chaincase on too, which was very vexing and will need proper alloy welding attention next time the motor is down to its elements.
But hey, where would we be without the odd hassle to undermine the joy of it all?

[RDfella]

I did find the bottom pivot bolt had lost a nut so there may have been some movement there.

Looks like your answer. Lot of strain trying to twist the 'box when under power and I'd bet your gearbox was twisting more than you'd like to know. Forces there are higher than one realises - even on my B31 a bolt through the lower chainguard lug is necessary to stop the box moving backwards under power and over-tensioning the primary chain as a result. Had to make a cam to lock the Norton box from doing the same on my Weslake. And both of those were with gearbox mtg bolts 'bloody tight'.


Add: another thought - how are the swing arm bushes?
Recall many years ago when the 1,000cc BMW came out (think they were 4 cyl) and one was giving it the beans as he was overtaking me in my car. These bikes had shaft drive and a single arm rear suspension. As the rider was alongside he changed gear and I was astonished by the back wheel movement as he did so - it stepped sideways around 2 inches, so again, lots of forces going on that we don't often consider.

[mikeb]

Radioing back in about this misaligned gearbox/sprocket. I took the primary and other bits off again to try a straightedge (which needs to be less than 1” thick) as in the pic. I set the rear axle parallel to the swingarm (by a measuring rod). The gearbox sprocket was a few degrees out of alignment but largely remedied when bottom gb pivot nut tightened when a little pressure down on the primary chain. I’m fairly confident its fairly true now, including the cush sprocket being in line with the clutch chain wheel. there could be something not 100% true about how the gearbox sits when free in the engine plates so next time I have the engine out I’ll investigate (if I remember!).

With the new chain and sprockets, the bike now runs so much more smoothly, especially noticeable at low speed.

RD – the swing arm has unity equipe bronze bushes that still feel tight. Tho a possible source of the wear could be the very loose silentblocs I replaced, and then the replacement silentblocs that soon also failed badly and were replaced too. Must have been NOS rubber.

I think rechecking this was worth doing to protect the new parts tho not much fun. Also a note to other non-experts to refit the left footpeg before fitting the primary case... unless you want lots of practice with disassembly

Thanks for the ideas and encouragement.