The ornamental design for a cap for aircompressor, as shown and described.
FIG. 1 is a front 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, the element depicted by means of broken lines being for illustrative purposes only and forming no part of the claimed design;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof, the element depicted by means of broken lines being for illustrative purposes only and forming no part of the claimed design;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a right side elevational view thereof; and,
FIG. 8 is a rear perspective view thereof, the element depicted by means of broken lines being for illustrative purposes only and forming no part of the claimed design.