The Minolta MC/MD register distance is about 43.72mm, whereas the Pentax one is 45.46mm. The difference between them is about 1.74mm.
- A M42-PK adapter (~7$)
- Some screwdrivers with small diameter.
- A drilling machine with 2 drills: 2mm and 3mm.
- A file.
2)Disassembling the mount
There are 8 screws, we need to unscrew the 4 long ones, because the 4 short ones hold the diaphragm actuator with the mount.
File off the ridge around the outside of the aperture ring.
In this above photo, the ridge were cut off.
Continue to unscrew the 4 screws of black ring.
Rotate the aluminum ring in counterclockwise direction. Because we don't need the mount spacer which have a thickness 4mm. Therefore we must adjust 4 - 1.74 = 2.26mm in height by rotating the ring.
3)Reassembling the new mount:
After rotating, we re-screw the black ring.
Drill the 4 holes respectively on the M42-PK adapter.
Note that: the red dot on the adapter will the same position with the diamond shape
Place the aperture ring in its position with the ball ring. Beware of the ball ring. It is very small, Do not lost it.
Re-screw 4 long screws on the new PK mount.
Voilà, the conversion is done.
We can do the same way with Minolta Rokkor-X 50mm f/1.2. This is the later version with MD mount. This lens has not the spacer mount, therefore we must file the lens cover as the image.
I converted the Kiron 28mm f/2 as the same way of the Rokkor-X 50mm. Its focal length is smaller, so it needed to file more precisely in order to focus to the infinity.
Here some results of Kiron 28mm with infinity focus: