Flat jeweller's screw-driver with a blade about 0.5mm wide. You can buy sets of these things for about $5.00. You loosen the retaining ring screws that are on the sides of the rear ring. The ring then just lifts off. Make sure the lens is lens side down. You then need to have a narrow (narrower than the width of the depression in the adapter, about 36mm, no wider. I used the open end of a black plastic film can because it just matched the diameter of the ring below the threads of the c-mount on the lens. I cut the end of the film cannister off just to the height of the rim on the lens, just below the threaded c-mount. Then I glued it in-place but I was careful to only apply adhesive to the wall of the rim on the lens below the threaded mount. The reason for this is that you want the aperture ring of the lens to be snug against the plastic of the film cannister, but not to bind it and not to be glued to it because the aperture ring still has to turn. If you cut the plastic ring to just the height of the lip of the lens mount rim (Just below the threads, see the other image of the lens in this thread) then when you attach the adapter to the lens, it will just touch it, forming a good mate. If you cut the thickness just right so there is some minor compression between it and the adapter, you don't even need to glue it because the adapter becomes the retainer. I leave the adapter on that lens (you can buy them for about $9.00 now) and if I need to use an adapter for another c-mount lens, I use a different adapter. Having said this, if someone can machine you a thick aluminum ring with set-screws in it, you can use thin piece of teflon cut into a washer between the aluminum ring and the aperture ring and it will provide a very nice, smooth fit.
