Gearbox Sprocket Nut Lockplate

Drill  a new hole.?   

Have you tried flipping the lock ring over?  It may need a bit of hammer work as I think it's slightly concave to suit the inside of the sprocket.

I also believe you don't have to do up the nut tight - so long as the lock ring is engaged - and the sprocket can be slightly loose on the splines.  But don't take my word for this until someone either confirms or says it's utter rubbish!

The F Neill book doesn't seem to mention the locking plate ( which seems odd). Haynes says to tighten the nut to 80 lb.ft. and then slacken enough to fit the screw in either one of the two tapped holes in the sprocket.  80 seems very tight, because the whole assembly is so stiff that even a small slackening will probably leave it almost if not entirely loose apart from the locking plate. It's a long time since I went in there but I'm fairly sure the nut wasn't very tight when the locking plate went on, just as Bob says.

I wonder if the 80lbs  is to ensure the bearing is fully seated in the case ?.

... reverse the plate as this moves the locking screw hole by half a flat. Easy to bend the plate to allow for the dish.

From an engineering point of view, I don't see why the gearbox sprocket must be tightened to 80 flbs as even at this crazy amount of torque, there could still be rotational movement, it will though stop any axial movement which may not be the best idea.  Ask yourself if the gears on their splines inside the gearbox are clamped into a fixed position?  Obviously not as you wouldn't be able to change gear!

Shame about your Laverda sprocket Andrew (I love those bikes) but I doubt very much that clamping the sprocket would have made any difference.

I use an 18" extension on a 1/2" drive socket and have never not been able to tweak the nut enough to line up with one or other of the tapped holes which should be at half a notch difference from each other so you'll never be more than half a hole out.

How close are things to lining up at the moment ? To be honest, taking into account the difficulty of keeping a box spanner square with your means of turning it, I'd be surprised if you're getting close to the suggested torque which probably means that you could turn the nut a fair bit further.

Mainshaft end float in the AMC box is controlled by the shaft being pulled into the inner case bearing and the sleeve gear being tight into the bearing. There is a hardened steel spacer between sprocket and bearing. The assembly has been designed to be done up tight.

I've just looked at my torque wrenches which I use for check-tightening. My Superslim only goes up to 50lbs and is 13" long. The big Norbar starts at 60 lbs and is 22 inches long. I think that you certainly need something between those two to reach 80 lbs. I find it rather difficult to assess torque with an adjustable...too many things moving about..

Various suggestions have been offered for the alignment of the lockplate , it can be turned over which is an option.    A further two choices would be to reverse the nut or reposition the sprocket on the splines or both of these.                                                             For the purpose of tightening my friend uses a substantial tube/ box spanner and tommy bar, this gives the benefit of using two hands and avoids the spanner riding off the nut . Drilling an additional hole in the sprocket isn't really practical as it's hardened and tightening the nut will not pull the main bearing into it's housing.

Lateral thinking.......Have you tried reversing the nut so that a different face is on the sprocket?

Alternatively.....Take the nut and rub it on some oiled fine grit Wet & Dry. This will easily take off enough off one face (probably 0.0001 thou) to allow the nut to rotate a fraction more than it did before.

In my greener days I used to hammer the nut round using a Brass drift until the holes lined up.