I recently obtained an old lens with a Kiev-88 mount, which I intend to use also on a Nikon F camera. Since the K88 to Nikon adapters I could find are very expensive, I got myself a K88 to Pentacon Six adapter and a Pentacon Six to Nikon F adapter, assuming that I should be able to simply stack them to be able to mount the lens. My problem is however, that for some reason, the diameter of the K88-P6 adapter is too large to fit into the P6-Nikon adapter. The difference is less than 1mm, but with metal on metal, not even brute force will join the two adapters.
To understand if the first adapter is too large or if the second adapter is too small, it would be great if by any chance someone could quote me the expected inner diameter of a Pentacon Six mount. Preferably from a technical specification, but if someone has a Pentacon Six and a slide caliper at hand, knowing the diameter of the mount opening on a real model would of course also help.