on my 660R the plate on the end of the chain adjuster (RHS) had come loose and been rubbing and grinding itself against the sprocket bolts!

I guess if I had been washing the bike more frequently, the damage might have been avoidable. For now a new plate is required!

This happened once before when the bike was less than 6 months old.

My question is what did I do wrong at my last chain adjustment. Both the rear axle and the locknut were torqued as per spec

Do you mean the plates at the end of the swingarm?..the ones that have a bolt adjuster with TWO locking nuts on each side?...

Do you mean the left hand side as you sit on the bike?

yes pal you are spot on on both counts

have you got both locking nuts on each adjuster?..if its slackening off after being torqued down maybe you could use a semi permanent thread lock..or give it some extra torque...

After loosing one plate (forgot to tighten lock nut), I am using one nylon nut now. For more than two years It seems OK. Probably I should replace the nuts just to be on safe side.

it has only happened twice in 12000 miles. First after delivery from dealer (2009) and second, after my last chain adjustment (a couple of weeks back)

now it could have been I didn't finish it off properly (strong possibility!)

Also maybe it is my technique:

I tighten the adjuster nut till hand tight then holding it in place with a spanner, I torque the lock nut against it.

If I don't hold the adjuster nut, it will move tighter when torquing the locknut. Maybe I should let this happen (??)

I was wondering why yamaha only specifies the torque figure for the lock nut and not the adjuster nut (one pushing against the plate)