An oil drain valve assembly is provided for draining oil from an engine crankcase such as in a small internal combustion engine. The drain valve assembly includes an adaptor, an outer sleeve assembly, and interlocking for permitting opening of the valve assembly by relatively limited counter-clockwise...http://www.google.es/patents/US5096158?utm_source=gb-gplus-sharePatente US5096158 - Oil drain valve assembly
