My friend told me I should use cling-wrap to protect the lens as I put a thin layer of Plumbers Epoxy Putty between the aluminum retaining ring and the M42 adapter. This thin layer also gave me the 1-2mm spacing I needed to get over the a raised ring on the stop down mechanism. The cling-wrap was a very good idea. I used two layers of the stuff. My fingers were very sticky with the epoxy putty. This kept the lens clean.