While looking over my bike this morning to see what kind of RD it had, I found the RD to be somewhat loose. I went to tighten it up (not sure on terminology, but it's the 6mm allen that the whole thing pivots on--the allen screws into the hanger piece that sits in the dropout), but found that it's actually a nut and bolt affair--the nut on the backside spins. I know this part needs to be a bit loose, so it can pivot; but I thought they were all tapped into the hanger? I'm going to try later on to tighten it; I'm hoping that the bolt has a shoulder to prevent it from being overtightened.

If you're referring to the mounting bolt at the top (see pic), it should be tight, so the derailleur won't fall of the hanger.

That's a 1984 part. No docs so I don't know if the nut is normal or added later...

Why not pull it off to see if the hanger is - or was - threaded?

I'll check that out (removing the bolt to see what the hanger looks like). I'm hoping to take a ride today; I'd rather mess with it after the ride (I have a very specific window to ride in). But that bolt is supposed to be tight? I though the derailer had to pivot a bit on that, as it takes up chain slack. If not, then mine is way too loose (and may explain why my shifting isn't optimal: I have to overshoot to shift, then back off to avoid noise--and lately has been upshifting suddenly on its own when under full power).

How did you know that it was a 1984 part? Or did I post the date code instead of p/n? [Might explain why I can't find the date code, eh?]

It is supposed to pivot, but there should be a pivot in the dérailleur mount itself. That bolt should be tight.

I looked up the part # on ADFC. Z501 was only sold in 1984, variations through 1986.

Cool, thanks.

Ok, looked again. I have no idea what sort of bolt this is. It definately goes through to the hanger (the piece that goes into the dropout and is screwed down), and there is a nut on the back of it. I can turn the bolt, and the nut will spin. I hold onto the nut with a wrench, and while it stops turning, the bolt can still spin freely. It does not appear to back out or tighten up.

While I was looking at it, I did find a rivet that was backing out of the parallel bit. I was able to push it back into place, more or less. I suspect the derailler is simply on its way out. Hopefully I can get a few more rides in while I figure out what to do (R&R the bike or buy a new or newer one).

Well, you got me. I have no idea. You might try asking in C&V how to get a RD-Z501 off.