Alignment

A long, long time ago I did the wheel alignment on my 2A Commando. At the time I must have been reasonably satisfied with the result because I have never been inspired to check it again. Last week however I took the back wheel off and discovered the circlip inside the brake drum had jumped out of position(!) which allowed the bearing to move and the brake drum to move away from the wheel, such that it was only in contact by the tips of the cast lugs in their cush drive slots!  Now I have it all rebuilt with new bearing/circlip etc, I thought I would give it a birthday and recheck wheel alignment. Big mistake.

I have now established that although front and rear wheels are perfectly parallel, the rear wheel appears to be 7mm nearer the drive side than it should be! In addition to that I notice that the outside of the rear sprocket teeth on the brake drum are showing signs of wear which is not evident on the inside of those teeth, suggesting the wheel is indeed offset towards the nearside.

The only thing I have touched over the intervening period has been the rear swing arm, which has been off to have new bushes and pin fitted, but I would swear it went back with the same spacers etc that came out. So it's a mystery to me.

As a simple man, all I can think of is adding a 7mm washer to the drive side and machining off the same amount from the chrome spacer on the speedo side? The problem I have is the chain run, which although obviously out of line as evidenced by the uneven wear, I think a 7mm misalignment would be visible? Alignment of gearbox sprocket to rear wheel to properly check this would require a strip down that I am not keen to embark on at the moment .

I have read on other threads however that other people have found similar discrepancies on their own bikes and have just decided to live with it. I would be relaxed about that as I have been riding it in blissful ignorance for almost 40 years - but I am concerned about the chain alignment. It may be however that this asymmetric wear is a symptom of the circlip jumped/bearing movement which is now resolved? So my current plan is to forget about trying the get the wheel alignment dead on - but keep an eye on the chain? Purists may faint, but I find the whole idea very difficult as soon as one considers the effect of the isolastic mounts on alignment principles?