Originally posted by hoojammyflip guys, surely the registration distance for Pentax and Olympus is different...
For simple lens mods, here's my table of registers:
44.7 mm - Exakta-Topcon bayonet
45.46mm - Pentax-Praktica, M42
45.5 mm - Petri, Contax-Yashica
46.0 mm - Olympus OM bayonet
46.5 mm - Nikon F-mount
To adapt Exakta lenses, I shave the adapter, not the lens. For Petri bayonets, I pull the old mount and glue-on a generic M42-PK adapter. For C/Y and OM, I taper the bayonet flags so they slide under the PK mount lugs. Nikons need a couple approaches: trim the aperture ring, or shim the base, or do nothing.
In practice with OM and Nikon, the register discrepancy Ain't No Big Thang. As
ripit said, you lose a little up close and can focus past infinity. And there's a way to finesse that: with Nikon pre-AI, I put 1mm spacers under the lens base (washers on the retaining screws). I haven't tried that with an OM lens yet, haven't felt the need.
On C/Y and OM and NI lenses, I may cut a slot in the lens base for the PK locking pin to engage, for a more secure fit. And on all these where I've just trimmed the lens base, I don't try to mount a lens weighing over ~600g. If I had replaced the base with a wide-flange M42-PK adapter, with that adapter held down with 3-54 screws, I would have no qualms about mounting a heavier lens. Hasn't happened yet, though.