Originally posted by PJ1 Pentax adapters are without doubt the best. But I have several OEM adapters (without the flange) and I just remove the spring. They work fine. If they have a tendency to unscrew from the lens, a little dab of Loctite will fix that.
Probably the easiest method. If the OEM adapters are in scarce supply, you can always get third party ones and wont feel as bad to remove the spring and secure to the back of the lens with locktite. Most M42 lenses are also wide enough at the rear that you'd have additional friction fit. Narrower lenses, I'd tend to use with the clip on as they don't even go past the adapter in width. (e.g Tak 35 or Mir 37/2.8)
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Long time ago) On a larger and heavier lens (eg Helios 40) I did drill a small indent where the locking pin would go to guarantee that I wouldn't accidently twist off the lens on the go. I do have a film camera where I leave an adapter clipped in for M42 lenses only
I use Sony now and have a handful of m42 adapters to fit out whatever m42 lenses I might fancy using on a particular outing.