Anything wrong with using a lens cap as mount?
 

I'm just (finally) getting ready to build my first adaptor and I'm wondering about the lens mount. Is there anything wrong with taking the back lens cap of a Nikon 50mm MF lens and putting a hole through it and using that lens as the lens mount to the adaptor? Sorry if this has been covered in old threads but I haven't found it. Would there maybe be problems with making sure its leveled properly?

Well, I think you've implied the answer to your question. Unless the lens cap is a massive metal thing, it's not going to be made to anywhere near the same tolerances as a true lens-mount assembly.

Except for some old screw-in examples, every cap in my collection is molded plastic. Even if you were happy with the accuracy of the lens's seating in such a kludge, the plastic would stand up to only a fraction of the wear-&-tear of a metal mount assembly.

Bob Hart
 

Plastic has a shape memory and aspires to return to that which it once was, a blob. Cut a hole in it and it may not necessarily stay round. With a hole in the centre large enough to pass the image, the lugs on the edge will more likely bend in and release the lens if it is a heavier one like a 28mm or 85mm f1.8 in the event of a shock load such as a tripod leg getting kicked.

The back cap also also does not have the locator pin to secure the lens so you will likely twist the lens out of the mount when focussing or working the aperture ring.

The Nikon FM2 mount rings are available as a camera part, along with the screws and back spring. The pin and release button you will have to make yourself.

When you say make...are you talking machining? Or will this be something I can put together with some basic tools. I have a workshop with just about everything you can imagine...but no machinist tools.

>>>>>The back cap also does not have the locator pin to secure the lens so you will likely twist the lens out of the mount when focussing<<<<

Yes! That is true. Workaround:
a) Change the grease in the focusing mechanics so they turn effortlessly (like cine primes) You will have to take apart the whole lens assembly. (Some lens are designed to stay together (the glass I mean) while you work the mechanics. Be careful WHERE the threads START and END!!!!!!!!!!!!!!!!!!!!!
You may have a hell of a time to put them back and have infinity where it should be!!!!!!!!!!!!!!! There are two of them on each of the seven I open recently.

and

b)mount a pin from the side to touch the lens (hold it in place and prevent turning) It will stop the lens (at touch) on a pre defined position ensuring you will have all marking lined up. (infinity and the rest)

The pin (I made) is 2mm dia, 7mm long, spring 4mm long. You will have to drill a hole in this pin and insert another one (as release button) from the side or... make a grove on the pin (ex 2.5mm down) and mount another release button (from the side)
I found plastic rear cups to hold very well (when inserted in a stronger round material) but they require precise machining. They will adjust to the larger hole (when pressed in) and keep a perfect circle shape.