Designed to Replace Fuel Line Fittings Without Damage to Fitting, Fuel Rail, and Fuel Line Reducing the Possibility of Fuel Leaks. 17mm socket works on Audi® A1, A3, A4, A6, A8, Volkswagen Passat®, and BMW® N54, N63, and S63 engines. 21mm socket works on Paccar® MX 11, and MX 13 engines. Use with 3/8� square drive.