Alignment of motor sprocket wtih clutch chainwheel

How closely does the motor sprocket have to be aligned with the clutch chain wheel on a chain driven primary?

In the pictorial in Classic Bike that showed Mick Hemmings doing this assembly, it said something like that the two have to be aligned; and had a picture of him using a straight edge to align them. I happened to have my primary cover off in the weekend (actually I had to take it off after the rotor nut ground its way through the primary cover and completely destroyed the cover, and caused collateral damage to the alternator - but that's another story and I'm not about to publicly admit to another piece of mechanical incompetance) and took the opportunity to measure the alignment.

When I put the primary together, I just relied upon the sprockets being aligned, and it looked like they were to the eye. I couldn't figure out how to measure it; I didn't have a metal ruler short enough to go into the gaps, and wasn't sure where to measure from. Last week I saw one thread on this site that suggested that because a chain inherently allows some sideways movement, the exact alignment is not critical (the same thread suggested a measuring device with a piece of wood and four screws, but I couldn't figure out how that worked).

During the weekend I did find a metal straight edge (actually a t square; but I only used the straight edge part of it) short enough to be put up against the both sides of the outside perimeter of the motor sprocket, but long enough to extend to the teeth on the clutch chain cover, so that I could measure how closely alligned the motor sprocket was aligned to the clutch chain cover (i.e. by placing the straight edge over the sprocket, I could extend the plane of the sprocket, and check if the plane was exactly in line with the outside teeth on the clutch chain cover). There is about 1.5-2 mm difference in the alignment of the two sprockets - the clutch chain wheel is further in the primary crankcase than the motor sprocket, so if I were to align the two exactly I would have to shim the clutch chaincase out by 1.5 to 2 mm - and of course order some more shims from the UK. I don't really want to have to disassemble the primary if I can avoid it; but I also don't want to ignore it and have my primary chain fail; or worse. Any advice gratefully accepted.

I found it alot easier to check with the chain off using a ruler across the engine sprocket and set the clutch wheel 1/2 way between crank end float,I shut my eyes and ARSEUMED the gear box is square to the motor, the 4 pins on a straight edge would tell you that more accuratly. The parts manual list a 0.036'' and 0.048'' shims so some where around 0.010 to 0'020'' must be the factory setting, ARSEUMED from the difference in size between the two shims. :idea:

I just did this on Friday.

I was over at my Norton guy's shop, where he happened to have a Combat on the lift and was assembling the primary. The bike came with two thick shims in the clutch assembly, so he double-checked the alignment - sure enough, two thick shims is what it took. Straight edge was a steel rule, laid across the face of the crank sprocket. I should fall right against the base of the teeth on the clutch drum, which it did when shimmed.

Bike also had a mixed clutch in it - 4 barnett plates and one bronze in an 850 basket. Going back all bronze.

I don't know how tolerant the chain is, but I would not allow 1.5 - 2 mm of offset on my bike.

It sounds as if someone has failed to put the shims back or possibly the location circlip is either worn or no longer seating correctly which should be investigated.

The shims do have a habit of sticking to the clutch on removal and are subsequently easily lost if you don't know they're there. I always wire mine with the locating spacer.

There's nothing special about the shims. I'm sure you must be able to find something similar in NZ, either from a bearing supplier or otherwise, give Ken McIntosh a ring. I have single cylinder parts sent to Europe from him.

Thank you gentlemen. It seems that the prudent thing to do is to pull the primary apart and do it properly; since my initial post I have been reflecting on the stresses that could be put on both the crankshaft and the gearbox by having the two shafts out of alignment; I am sure that these things are often not perfect (like the point made about the potential inerent misalignment of the gearbox), but clearly I am inviting trouble on a number of potential fronts if I don't do it again properly, and leave additional potential stresses. I can't remember whether there are currently any shims behind the clutch cluster; but the "someone" in the tale is me... I do have a recollection of looking at the circlip, and not noticing any wear, so I hope that is not the culprit.

Luckily British Spares in Wellington appear to have shims in stock; I have ordered two large ones and one small one, and hopefully a combination of probably two out of the three will work, and hopefully they will arrive before the weekend.

Chris, it occurs to me that the other important thing to check is that the engine sprocket is going fully home on the crank taper. If your mainshaft nut has been loose then there is a risk that the shaft or woodruff key have become damaged and are preventing the sprocket from seating properly.

The best way to check this is with some engineer's blue on the taper. If the damage is not severe then it can be lapped with some fine grinding paste.

I've never yet seen a Commando clutch without any shims behind it.