The ornamental design for a cap for aircompressor, as shown and described.
FIG. 1 is a front and upper perspective view of a cap for air compressor showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a right side elevational view thereof;
FIG. 8 is a rear perspective view thereof;
FIG. 9 is a front and bottom perspective view thereof; and,
FIG. 10 is a front and upper view thereof, the openings and the protrusions and the bulges depicted by means of broken lines being for illustrative purposes only and forming no part of the claimed design.