A few days ago on another forum, there was a discussion about using a Helios 44 lens on a Nikon. You can buy adaptors to fit M42 lenses to Nikons but as the Nikon register distance is 1.04mm longer than M42, it is not possible to get infinity focus (unless you use an adaptor with a lens element).
As I have three of these lenses, I decided to see if I could convert one to Nikon F mount.
I removed the existing mount/back plate and measured its thickness at 6.6mm. Then I took a Nikon F mount off of an old, broken, third party Nikon fit lens. After filing down a ridge on the back, this measured 4mm thick so I would now be 2.6mm closer than before. To get to the correct distance, I needed a spacer 1.56mm thick. I had some 1.5mm black plastic which was close enough so I made a spacer which was a bit larger diameter than the mount in order for it to act in its other capacity as a flange to stop the aperture ring from falling off.
Anyway, I proved to myself that it can be done.
And then today I read that these were made in Nikon F and Pentax K mounts so I needn't have bothered!