BoxoUSA 3/8" drive metric spark plug sockets come in two sizes, 16mm and 21mm, each with a mirror chrome finish and a rubber retainer insert that holds the plug during removal and installation. They are made for spark plug changes in automotive and small engine work, with a knurled ring for turning the socket by hand to start the threads. An external hex on each socket lets it be turned with a wrench where a ratchet will not fit.