I have a Raleigh Twenty with original BB and cottered crankset. I use this bike for daily commuting. Last weekend I noticed some knock/play in the crank arms, so I tightened the left cup a little to get it tight without binding. I have done this thing a few times in the past as well, but paid no further attention to the matter.

Last night I was looking at something else and pulled on the crankarms, and thought I felt a bit of play. So I tested it, and sure enough the same amount of play/knock that I removed last week is back again!

To adjust, I loosen the left lockring, tighten the cup, and tighten the lockring again, by tapping with a hammer and drift.

What is going on here? Is the left cup moving over time? I check its relative position, and it seems the same as what I left it last week. The right cup is difficult to access due to the difficulty of removing the cotter pins. I once tried to unscrew the right cup, but it was immovable, and I assume it still is. But perhaps it is moving? Or can it be wear? Seems unlikely.