I have been using Diesel motors ( S.T. / Olly T / PAW ETA etc ) for over 30 years and have a tip.

Find a nut that fits the comp screw thread, then find a course type "star washer" that is a neat fit.
Put the star washer between the head and the nut, pinch the nut down just enough to slightly compress the star washer and also give the correct amount of stiffness to the comp screw.

This allows the comp screw to be adjusted but the stiffness stops it backing off.
I have motors that have been setup like this for years without having to mess with the nut tension at all, once set.

p.s. - surely there is no likelihood of stuff in the gap above the contra piston igniting !! - at least I have never heard of it.