Originally posted by Wheatfield I did precisely what you want with a Takumar 17mm fisheye lens.
I took a pentax M42-K adaptor and removed the spring. I then put the adaptor onto the lens and, using the screw that was supposed to hold the spring in place, I jammed the adaptor to the lens.
A little careful measuring and a bit of work with a Dremel tool to put a small notch in the lens mount for the lock pin to engage, and I now have a Super Takumar 17mm fisheye in K mount.
A lot of M42 lenses won't allow this because they are enough smaller than the K mount that you can't get the pin to engage.
I did the same thing with my third-party adapter (the kind that allows infinity focus). I removed the spring and I have it mounted on the only M42 lens I use with my K10D, my old Super Takumar 50mm, f/1.4 from my Spotmatic.
I didn't bother notching or drilling the lens for the locking pin. I have found that friction is more than enough to keep the lens in place. I never had a problem with the lens falling off my Spotmatic and it doesn't come loose in the adapter. Nor does the adapter seem prone to falling off the camera.
I put the adapter on the lens as tightly as I could. With the combo off the camera, I held the lens in one hand and used the flat side of a screwdriver in the other hand to screw the adapter onto the lens. I can still get it off, if I need to, but it doesn't loosen when I dismount the lens-adapter combo from the camera.