Coarse rags or towels also help to prevent ball bearings from rolling away.
I forgot: is the 2/50 pentax the M42 or the PK mount? The PK mount one has a tiny, tiny ball bearing that GOES FLYING when you take off that aperture ring. The older, M42 lens has a LARGE ball bearing, maybe 2 ml even, and is easier to manage.
If my walls had memory and vocal chords they would be able to recant the swearing that I have done over the years about these utter frustrations. - David Lyga