Originally posted by boriscleto The spring clip holds the adapter in the camera. If you remove it all that is holding it in is friction. Members here who have lots of M42 lenses buy the cheap adapters in bulk and keep them on the lenses permanently. You can also cut a small slot in the adapter so that the cameras lens release system can grab onto it. If RioRico sees this thread he'll give you his whole spiel.
Just the short spiel. No long spielunking today. Cave worms ate my brain.
With cheap clone narrow-flange Bower-type M42-PK adapters, always remove the screw and clip OR YOU WILL GO APESH!T! After removal, I use a PK rear lens cap as a wrench to tighten the adapter on the lens. If I'm paranoid, or intend to leave the adapter in place awhile, I may put a wee tiny drop of contact cement on the threads. And I may use a dremel to cut a slot (about 1.5mm deep and wide and 6mm long) or drill a hole (about 1.5mm deep and wide) for the PK lens-lock pin to engage.
For close work, whether portraits or product shots or anywhere infinity doesn't matter, I like using a safe cheap wide-flange NIF (no infinity focus) adapter. No clip, no stress, no swearing. Using such on any screwmount lens forces me to see differently, to look for subjects within the reduced focus range. It's just another discipline.