Wow! Thanks for all your replies. I'll just go ahead and reply to all your posts in this one message.
True, the bike is 20 years old (how did THAT happen?), but it only has 26,675 miles on it. It's been very well taken care of with regular oil & filter changes with quality products. It's been ridden spiritly but not flogged. It's never had any problems that couldn't be fixed by a proper tuning or a MF battery.
The bolt that holds the gear on isn't as worn as it looked. I'm guessing it just looks that way because of the lighting and the camera. In fact, this is the first time the engine has ever been opened-up.
I, too, suspect some washer wear. The copper inboard washer looks pretty good and I really doubt it's lost 2mm of material. I figure I'll go ahead and replace it while I'm in there, even if it cost $ 6.00
As for the taper washer, thanks Cegodsey for pointing out that the gear may be intended to move. Even though the pic doesn't show it well, there still is quite a bit of taper left to it. But as has been my motto on this project, "While I'm in there, I may as well replace it."
Seeing as this gear may well be intended to move, perhaps it's just getting hung-up a bit. When it gets free I get that smack. Perhaps I should replace the needle bearing that goes between the gear and the counter-balancer shaft, too?
As for the smack sound it makes, when it decides to happen it's only on the first start of the day. I've gone ahead and bought a set of ACCT springs from TOC for the rebuild, so that should take the ACCTs out of the smack picture.
Oh, and thanks for the compliment on the pics. Once I found the camera's macro mode, I got much better results.
As always, I look forward to reading your thoughts on all this.