3/8" drive with black oxide finish. 17mm socket works on Audi A1, A3, A4, A6, A8, Volkswagen Passat, and BMW N54, N63, and S63 engines. Also works on Ford Power Stroke Engines. 19mm is found on 2011 and older diesel engines plus Cummins ISB engines. 21mm socket works on Paccar MX 11, and MX 13 engines. 22mm is used on the newest ISX Cummins motors. 24mm used on NOx and soot sensors.