The three components you are viewing makeup the upper part of a pump.  All components use standard SAE size O-rings.  Sealing has been tested, as well as the pump.  It is not a high-flowrate pump.  It is sized for a standard hobby motor.  You will need some sort of sleeve to fit into the pump housing in order to tighten the clearances so the pump will work.  It was originally designed to bolt to the side of a tank.  I cannot remember what size O-rings I used, but they were almost all 200 series.