I picked up a tired copy of a Rikenon-P 28-100 f/4 lens recently. I tightened a few handy screws around the edges, but the front element was shifting forward and back in the zoom assembly, leading to random focus. Given the price I paid and its near-useless condition, I decided to go into the lens and find the problem, and hopefully solve it. Four screws allowed the front portion to come off - one was stuck then stripped, so gentle application of my drill "solved" that problem. Once removed I found that the front portion of the lens was not the problem: the zoom mechanism that it attached to was the source of the troublesome wobble. I tried several ways to reach the heads of several loose screws which I could see through the assembly from the backside, but had no luck - the "access panel" was held by tiny flathead screws (I've misplaced that screwdriver), and they were securely glued in any case. After a few other attempts to break in, I went to plan 'B': a drop of super glue on the backside of each screw. Once set, that trick cured the wobble for good.
Reassembly was another adventure: two small metal inserts needed to be reattached, one to keep the lens from focusing beyond infinity and the other to prevent unscrewing the front element beyond minimum focus. I was successful at first (another drop of glue would need to replace the drilled-out screw), but something was not aligned right - focus and/or zoom were grinding near infinity and 100mm. One of the inserts was not behaving, so off came the front piece again. I decided that focus past infinity was not a real problem as it could only go so far (though I need to check that elements are not making contact!), so I left out that insert. Once the minimum-focus support was screwed down the lens worked again – better than ever since the wobble was gone. Whoopee!
This was quite demanding work! It took several attempts to get the front end to match properly with the rest of the lens, necessary to get focus where the markings indicated. The screws I could not undo also locked the focus ring in place, so I couldn’t ‘fake’ this. It was spooky dealing with the tiny screws, and I had to do it several times - but I had done it back in the ‘80s with a useless lens, and given enough patience the results were worthwhile. Other than its 605g weight this is a fun and flexible manual-focus lens,