Abstract: An oil separator cartridge includes a cylindrical spin-on housing having opposite inlet and outlet ends connected by an intermediate wall with the inlet end including a threaded aperture threadably mountable on an engine oil output tube having a smooth cylindrical outer end surface; a centrifugal separator rotor unit is mounted on an axially shiftable shaft in the housing for rotation and axial reciprocation for receiving oil from the oil output tube. A coil spring mounted in the housing urges the axially shiftable shaft and the centrifugal separator rotor unit toward the inlet with a force of sufficient strength to normally overcome the axial force exerted on the rotor unit by oil under normal pressure in the oil output tube; however, the coil spring permits the rotor unit to axially move away from the inlet in response to pressure surges above said normal pressure in the oil output tube to cushion the effect of such pressure surges on the cartridge.