The Machine, in the embodiment shown, is an air (or gas) compressor having a housing with parallel, intersecting bores in which are rotatably journaled coacting, meshing helical rotors. The rotors, male and female, are of the asymmetrical type. Confronting concave and convex surfaces of portions of the grooves and lobes of the rotors, for having different arcuate conformations along principal lengths thereof, define a void of varying width therebetween. However, other, minor extents of the confronting surfaces are defined of a common arc and, therefore, nestably conform with each other (a) to reduce contact stress therebetween, (b) to improve rotor-to-rotor sealing, and (c) to accommodate therebetween a significant film of oil.