The Y-axis leadscrew and nut were shot on the 8520 so I ordered acme rod and a bronze nut from McMaster-Carr and created a new screw on my old SB 10K lathe. (Clausing will sell the bronze nut still, but they don't sell the leadscrew anymore. Stock leadscrew is 11/16" and I can't get that anywhere, so easier to make both out of 3/4" from McMaster. There's a guide online to do this, so not my idea.) The screw came out well. The bronze nut is next. I have it partially turned down, and now it's on my benchtop mill to get it down to size. That will take me a while on the little mill.
Almost every bearing in the machine was bad. I've replaced almost all and I'm removing years of gunk. I also cleared out the bug nest from the quill gears. It's got a crappy paintjob and it's missing some small parts, but it should be usable soon.
TBD if my cleanup will help movement on the Y-axis or not. I suspect it needs the ways scraped/ground. That's probably more $ than the thing is worth. If it was a nicer one, I'd probably spend the extra money.