1. A SCREW COMPRESSOR COMPRISING A PAIR OF MATING HELICAL ROTORS, A DUCTILE METALLIC COATING ON ONE OF SAID ROTORS HAVING A PLURALITY OF RAISED PROJECTING PORTIONS ON THE OUTER SURFACE THEREOF PRODUCING INITIAL INTERFERENCE BETWEEN SAID ROTORS, THE MAXIMUM THICKNESS OF SAID COATING AT SAID RAISED PORTIONS BEING GREATER THAN THE CLEARANCE BETWEEN SAID ROTORS AND THE MINIMUM THICKNESS OF SAID COATING BETWEEN SAID RAISED PORTIONS BEING LESS THAN THE CLEARANCE BETWEEN SAID ROTORS, SAID SURFACE POSSESSING SUFFICIENT SPACING BETWEEN THE RAISED PORTIONS THEREOF TO PERMIT PLASTIC FLOW OF THE COATING MATERIAL IN SAID RAISED PORTIONS INTO THE SPACED BETWEEN SAID RAISED PORTIONS UNDER PHYSICAL STRESS CAUSED BY A RELATIVE MOVEMENT OF SAID ROTORS DURING RUN-IN OPERATION TO ELIMINATE SAID INTERFERENCE AND TO PRODUCE SUBSTANTIALLY SMOOTH MATING SURFACES HAVING NO LEAKAGE PATHS.