The Distributor insulator block set is used to insulate the incoming wire from the Coil to the Distributor. Always make sure this block is in place and in good condition when servicing your Distributor. This insulator block is for Delco Distributor housings with a .300" diameter hole w/locator key. Fits most cast iron Delco Distributors in AC models: B,IB,C,CA,RC, all "W" series Tractors, and "D" series Tractors- D10-D19.