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.