I see a lot of M42 lenses, and generally want some test shots to prove that they're working before I sell them on. I've got an M42-Nikon adapter with a corrective lens, of course, and it works well for most lenses. But every now and again you come across one that is M42 auto only, with the rear stopdown pin but no other way to stop the lens down. It's usually a sign of a cheap lens, but some surprisingly good lenses have variants without a stop-down switch, including some Helios and other Russian models, some Tessar and other lenses for Praktica, etc.
In the past I've tried taping the button down, which sort of works but the tape tends to come off pretty quickly, and may stop you from screwing the lens all the way in.
What I finally came up with (and can't believe I didn't think of it sooner) is simply a shim, a ring of hard black 1.4mm plasticard (plastic sheet) cut so that it fits in the mount around the corrective lens. As you screw the lens in the pin is pushed in by the shim, after that the aperture is manually controlled. A single shim will push the pin in far enough to stop most lenses down to F8 or so, two will push the pin all the way in. I'd recommend going with one unless you really need both, to avoid strain on the pin etc.
I made them very easily, the only tools needed are a pair of sharp dividers, a ruler or calipers for measurement, a sharp knife or scissors for trimming, and possibly sandpaper to get rid of any loose bits of plastic. Start off by measuring the inner diameter of the tube (which will be around 40mm) and the diameter of the lens and its surround - in my case 32mm. Then use the compass to scribe two concentric circles in the plastic - and keep going until you are nearly all the way through the plastic. After that the card outside can be got rid of fairly easily, just cut a slit to get it started, then bend it back and forth around the rim until it separates. The inner part is a bit more difficult, but if you're careful you can separate the ring without breaking it. I'd recommend sanding off any loose bits of plastic, but I forgot to buy sandpaper so I wasn't able to do it for these pictures. The last picture shows the lens (a cheapo 135mm) stopping down to F22 with two shims, before I could only use it at full aperture.
Cutting the rings
The (fairly crude) result
Fitted in the converter
Stopping down the lens
Later - I forgot to say that I did it this way because I already had the plasticard - if I hadn't I would have probably found a plastic box of suitable thickness and started from that, or looked for ready-made shim rings of some sort. Mostly, this is just to show that shim rings are a solution to this particular problem.