Pipe coupling and drain plug for engine oil change

A pipe coupling used for changing oil in an engine pan with an oil drain port, comprising:

• a drain plug to be attached to said oil pan,
• said drain plug having
• a drain plug main body with one end to be attached to said oil drain port of said engine pan, the other end projecting outward from said oil pan, a recess formed in an outer surface of said other end, and an axial hole capable of communicating with an interior of said oil pan and forming an oil flow path,
• a valve holder fixed, in said axial hole, at a front position of said other end, and having an end face facing outward and an inclined surface positioned on an axially opposite side to said end face,
• a valve body urged against and supported by the inclined surface and capable of closing said flow path, and
• a first seal member which is fitted to an inclined surface of the valve holder or the valve body and which seals a space between an inclined surface of the valve holder and the valve body wherein said drain plug main body holds the valve holder at a position at which the end face does not project beyond said other end and the valve holder holds the valve body at a position at which the valve body does not project beyond the end face, thereby preventing the valve body from being separated from the inclined surface when hit by a stone or the like,
• said pipe coupling further comprising a socket which can be connected to said drain plug, said socket having
• a socket main body with one end that can be mounted on said other end of said drain plug, and an axial hole capable of communicating with said axial hole in said plug main body and forming an oil flow path,
• a second seal member arranged in said axial hole of said socket main body to engage with said end face of said valve holder in a fluid tight manner when one end of said socket main body is mounted on said other end of said drain plug,
• a plurality of locking members provided to said socket main body and capable of engaging with said recess when said one end of said socket main body is mounted on said other end of said drain plug, and
• a sleeve mounted on an outer surface of said socket main body to be movable in an axial direction, and capable of holding a state wherein said locking member engages in said recess,
• wherein said valve body opens said oil flow path when one end of said socket main body is mounted on said other end of said drain plug.