Well, I've discovered some additional reasons as to why the Ai 85mm f2 gets such a bad reputation. I got mine as a lightweight alternative to the 105mm 2.5, 85mm 1.8 H.C and the 85mm 1.4 Ais. The focus was stiff, with the rear mount screws locked down tight, typical Ai stuff. I didn't want to break into it since I got it for a good deal and it was quite clean. Well, shooting at a wedding at the reception I launched it onto the floor, hardwood and it took more of a slide then drop, I was on my knees changing lenses and that one got away. Quick check was severely restrained focusing, so into the guts I went over this last week.
So I daub on the lube on all the right spots, let it sit for a few days to soak in and start opening it up. Inside, WOW. Very similar to the Ai 135mm 2.8, which is to say, Trouble. So many points of design, manufacturing, and assembly that conspire to fail, or at least not work very well.
I'll bet that Nikon had a few heads rolling when the Ai hit the fan. Seems to me that with the early Nikkors, up to the mid 70's, the metal barrelled lenses had exquisite design, manufacturing, and assembly. Yes they are heavy, and yes they have a lot of screws (slotted and thin, with soft metal so strips are common), but most of the time they are built to last. They probably cost Nikon a lot to make, and no doubt took a fair bit of time to make.
Fast forward to the mid to late 70's. Competition increases, and sales go up, and I think this put pressure to make a more simplified design for the manufacturing cost side. The internals of the Ai series reflects this, using smaller helicoid rings and a more standardized apeture assembly. But the Ai was a failure IMO. They cut costs on the raw side, but the design of the internals were excessively fiddly, causing sample variation from poor workmanship. (I don't blame the worker, putting together a 135mm 2.8 is and was hell, I'd go nuts if it did that lens every day for a week.) I have no doubt that many Ai lenses were returned to be fixed, and I'd say that could be even worse, the Ai internals made for regular adjustments or repairs nearly impossible to do correctly.
By the Ais series, Nikon has fixed the problem. Many parts are stamped instead of milled, fixed into place by lighter screws and secured by the infamous Red Glue. The internal design more closely mimics the early metal barrel series, but with subtle cost savings in materials and parts. Top of the line lenses such as the 85mm 1.4 resemble the early lenses very closely.
So my advice, having been into the depths of Ai. If you are looking for a specific lens, if its made in Ais, that is the most preferred. If there is a choice between a more worn Ais and a 'mint but stiff' Ai, I think I'd still take the Ais. A cheap Ai that has a stiff focus may seem like a good deal but there is way more to go wrong inside. If you get a CLA on it there may not be that great of an improvement.
So, I did fix the little 85mm. Took 2 weeks, and it still has a bit of a slow section of focus which I'm sure will work out in time. Nice and sharp as well. I sure wish I'd waited for an Ais version though..