AJS & Matchless Owners Club

Absolutely spot-on, Paul (as usual!). I really appreciate the advice here and I've very glad I didn't ride it any further. Would have resulted in an expensive failure, I reckon...

I've checked: there is NO spacer fitted and the sprocket is the wrong way round. It was just a coincidence that the sprocket happened to nip up on the crank at more or less the right distance to align with the clutch, otherwise I would have noticed something was wrong when I changed the clutch centre!

It looks like the previous owner has managed to lose the spacer, as there isn't one anywhere in my parts box.

So I need a 041091 spacer, basically. It'll need to be approx 3/16" thick to get everything to line up. ID is approx 7/8", OD approx 1 1/4"

Anyone got one? Anyone know if 023066 is the same thing??

I had no spacer and I don't think there should be one just the shims that are listed as reqd. I had two shims about 10 thou thick each so I just put them back behind the sprocket and it appears to line up fine. A lot better than the primary chain grinding chucks out of the rotor

Yes, there are a couple of 10 thou shims in there too, but I'm pretty certain it needs a spacer as well. Otherwise there's nothing for the sprocket to tighten up against: it's about 3/16" short.
The sprocket definitely can't be fitted the other way round, as the chain fouls the rotor.

I suppose what could have happened is that the old sprocket didn't need a spacer, but the replacement one does. Maybe there was an undocumented change at some point? These things seem to happen with Lightweights. That might explain why the previous owner didn't fit one?

I think I need to track down a spacer from somewhere...

[edit]
The sprocket does tighten up without the spacer, but it's then misaligned with the clutch sprocket by approx 2-3mm (by sight, I haven't measured it yet). So it needs a spacer of that sort of thickness - so quite a bit thinner than the 3/16" I first thought.

Mine seems fine without any spacer but I suppose horses for courses!!! Just make sure the clutch unit centre nut is fully tightened as well as these have a habit of coming loose. I use Loctite on all such nuts as an added safeguard. What happens is that when you tighten the crank nut it tightens the sprocket tight up against the inner track of the outer crank bearing and pulls the crank towards the rotor end removing the end play (the crank is a tight sliding fit in those two bearings that end) This leaves the 20 thou or so gap at the other end (the timing bush end) to allow for any expansion when the engine gets hot.

OK, I think I came up with a fix.

I checked the misalignment of the sprockets and it's ~3mm. The required spacer is 0.9" ID, 1.25" OD. I couldn't find shim washers anywhere that size (and it seems it costs £10-15 each for custom-made shims). In the end, I ordered a packet of DIN 988 32mm x 22mm x 0.5mm shim washers, as these are cheap and widely available. I used a die grinder to increase the ID to 0.9".

The result is the sprockets now line up, the crank end float is gone and there is plenty of clearance both sides of the chain.





I'm still not 100% happy: the splines on the crank sprocket don't overlap the crank splines entirely and the alternator rotor is now slightly further out than it should be (so the crank nut only just engages fully with the thread). I suspect the crank sprocket is "wrong". Perhaps it's from a different bike? Anyway, it's all I've got for now. I don't think I can get a replacement duplex sprocket easily.

Let's hope it holds together...

As long as the rotor doesn't hit the casing when all back together you should be ok. Maybe turn that nut round as it looks like the flat side of the nut is facing outwards (just me being finicky!!). Have you got a wavy washer under that nut as it is this that will provide some sort of tension to stop the nut undoing? But looking at yours and the position of the nut on the shaft, it looks about the same as mine(and I only have the two shims behind the sprocket) and is about right if you have a washer under the nut. I doubt very much the sprocket is wrong it looks exactly the same as mine.

And another one showing the primary chain and rotor etc

Thanks, Paul. That's reassuring! Your sprocket looks pretty much identical to mine and your nut also looks pretty much flush with the end of the crankshaft. Still a mystery as to why I need a spacer, but hey-ho.

There is method in my madness: I took the washer out from behind the crank nut (because there wasn't quite enough thread) and turned the nut around so it doesn't gouge into the rotor. Everything seems to clear the cover. It's nice & tight and I've used thread lock - so it ought to be OK.

PS: is there a method to get the nut really tight? I'm using a clutch locking plate to do this, but I'm slightly nervous about how much load I'm putting on the chain when tightening the nut. Anyway, it's as tight as I dare...

TBH I can't remember how I did mine now. I haven't got a proper clutch tool so knowing me I probably just wedged something across the clutch basket and tightened it up that way with the chain on

Revisiting this thread from a while ago: I now understand why my bike needs the extra spacer on the crank sprocket.

I've got the wrong mainshaft in the gearbox! Now that I've had the gearbox to bits, I found that my mainshaft is the longer one from a 350 (p/n 041703). This would explain why the clutch sprocket doesn't line up with the crank.

Fortunately I've managed to track down the correct mainshaft (p/n 043058), so when I get round to rebuilding the engine I can sort this out and hopefully discard my home-made spacer...

[edit: though now I checked the parts list and it says that 041703 is the correct mainshaft for a 1966 250 CSR. Argh!!]