A hammer union comprises a thread end, a nut end and a hammer nut which, when cinched up, compresses a composite seal assembly thereby preventing leakage. The seal assembly comprises a metal insert of a size to pass into the seal groove of conventional hammer unions and a small seal acting between the insert, the thread end and the nut end. The small seal is conveniently an O-ring and is of a size that is compressed when the seal assembly is inserted into the thread end of the hammer union, thereby preventing the seal assembly from falling out of the threaded end when it is inverted.