A compressor comprising: a housing including a suction inlet and a discharge outlet; a compression mechanism configured to receive a vapor at the suction inlet and to provide compressed vapor to the discharge outlet; a volume index valve arranged near the discharge outlet, the volume index valve including a piston positioned within a hollow chamber and configured to move between a closed position and an open position to provide a bypass flow path from an intermediate portion of the compression mechanism to the discharge outlet, the piston being configured to move within the chamber automatically in response to the operating pressures of the vapor within the compressor, wherein the hollow chamber includes an integrally formed first portion having a first cross-sectional area and second portion having a second cross-sectional area larger than the first cross-sectional area, the first portion being positioned adjacent the compression mechanism and the second portion being adjacent the housing, wherein a cover mounted to the housing overlaps an end of the second portion of the hollow chamber, wherein the piston includes a first section arranged within the first portion of the hollow chamber and having a cross-sectional area complementary thereto, and a second section arranged within the second portion of the hollow chamber and having a cross-sectional area substantially complementary thereto, characterised in that the cover includes at least one flange extending into the second portion of the hollow chamber to define a third portion of the chamber having a cross-sectional area smaller than the cross-sectional area of the second portion of the hollow chamber, but larger than the cross-sectional area of the first portion of the hollow chamber, wherein the piston further includes a third section integrally formed with the second section, the third section being generally arranged within the third portion of the hollow chamber and having a cross-sectional area generally equal thereto, wherein the piston further includes a through hole configured to transmit a discharge pressure acting on a free end of the piston, into the portion of the hollow chamber between the cover and a second, opposite end of the piston. a housing including a suction inlet and a discharge outlet; a compression mechanism configured to receive a vapor at the suction inlet and to provide compressed vapor to the discharge outlet; a volume index valve arranged near the discharge outlet, the volume index valve including a piston positioned within a hollow chamber and configured to move between a closed position and an open position to provide a bypass flow path from an intermediate portion of the compression mechanism to the discharge outlet, the piston being configured to move within the chamber automatically in response to the operating pressures of the vapor within the compressor, wherein the hollow chamber includes an integrally formed first portion having a first cross-sectional area and second portion having a second cross-sectional area larger than the first cross-sectional area, the first portion being positioned adjacent the compression mechanism and the second portion being adjacent the housing, wherein a cover mounted to the housing overlaps an end of the second portion of the hollow chamber, wherein the piston includes a first section arranged within the first portion of the hollow chamber and having a cross-sectional area complementary thereto, and a second section arranged within the second portion of the hollow chamber and having a cross-sectional area substantially complementary thereto, characterised in that the cover includes at least one flange extending into the second portion of the hollow chamber to define a third portion of the chamber having a cross-sectional area smaller than the cross-sectional area of the second portion of the hollow chamber, but larger than the cross-sectional area of the first portion of the hollow chamber, wherein the piston further includes a third section integrally formed with the second section, the third section being generally arranged within the third portion of the hollow chamber and having a cross-sectional area generally equal thereto, wherein the piston further includes a through hole configured to transmit a discharge pressure acting on a free end of the piston, into the portion of the hollow chamber between the cover and a second, opposite end of the piston.