These engine zinc anodes come complete with a brass plug and are compatible with Ford Lehman engine models and all Caterpillar diesel engines. For optimal performance, zincs should be replaced every season or after two-thirds of the zinc has eroded, whichever occurs first. Here are the specifications for the E-1D Engine Zinc Anode: * **Size:** 3/8" NPT x 1-3/4" * **Pencil Length:** 1-3/4" * **Diameter:** 1/2" * **Diameter:** 1/2" * **Plug NPT:** 3/8" * Please note that the picture provided is of a larger size and is for reference only. Our anodes are made to current Mil Spec A-18001K.