These are pre cut zinc engine anodes complete with brass plug and zinc. marine engine zinc Anode for all Caterpillar diesel engines. Zincs should be replaced every season or after 2/3 of zinc has eroded, which ever happens first. E-3 ENGINE ZINC ANODE 3/4"NPT x 2" Engine pencil zincs-complete with end plug zinc pencil length:2" diameter: 3/4" Wrench size: 1-1/16" plug n.p.t = 3/4" Picture is of a larger size-for reference