Tightening a standard screw with a deformable flat washer or o-ring can crush and tear the washer or o-ring. Tighten too little and the assembly will loosen quickly during use. In either case, it is impossible to obtain a proper tightening of the screw head with the surface, there will always be rubber in-between. Over time, this rubber mount will loosen, and require regular re-tightening. After so many re-tightenings, or after servicing of the equipment, the o-ring or washer should be inspected and replaced.