Metering drain plug

Claim 1

In combination: an engine oil pan having a tubular fitting oriented to define a vertical escape passage for gravity discharge of dirty oil from the pan to a subjacent collection vessel; the inner surface of the tubular fitting being internally threaded along its length; an externally threaded plug adapted to be screwed upwardly into and downwardly out of the threaded fitting between a first position fully detached from the fitting and a second position sealing the liquid passage; said plug having an outwardly radiating flange at its lower end, said flange having an upper face that aligns axially with the lower edge of the tubular fitting; a resilient compressible sealing ring (22) carried on said flange for sealing engagement with the lower edge of the fitting when the plug is screwed into the fitting; the threaded surface of the plug having a flat (34) machined therein to define an auxiliary passage communicating the space above the plug with the space circumscribed by the sealing ring, whereby the plug can be partially unscrewed from the threaded fitting to permit discharge of metered liquid quantities from the oil pan; the upper edge of the tubular fitting being no higher than the interior bottom surface of the pan, whereby water and sludge impurity on the pan surface can readily reach the auxiliary passage.