Abstract: The design and construction of present oil pan assemblies have caused increased manufacturing cost, reduced seal life and reduced engine reliability. The present invention overcomes these oil pan assembly construction problems by using an oil pan assembly including a thin walled structure having a drain port formed therein. The thin walled structure further has a frusto-conical contoured lip around the drain port being concave toward the inside surface. The frusto-conical contoured lip has a sealing surface on the portion of the outside surface forming the frusto-conical contoured lip. A reinforcing member having a threaded hole therein and a conically contoured support surface at one end of the threaded hole is positioned so that the threaded hole is generally centered about the drain port. The support surface is located in supporting relationship to the frusto-conical lip. A drain plug having a threaded end and a sealing surface also includes an O-ring seal removably attached to the drain plug.