Groco Seacock Backing BlockUse the Groco Seacock Backing Block to improve flanged seacock installation. Groco backing blocks are made from an environmentally-friendly PVC foam specially formulated for marine applications. The rigid 1/2" thickness provides secure installation but is flexible enough to conform to minor hull curvature. To install, press threaded brass inserts into the hull side of the backing block into the holes that match the seacock flange. Each size fits multiple seacock sizes; Will not swell or absorb moisture; Specifications: Model: BB-1; Fits Seacock Sizes 750 and 1000; Backing Block Diameter: 4.29"; Insert Threads: 5/16-18; Model: BB-2; Fits Seacock Sizes 1250 and 1500; Backing Block Diameter: 5.38"; Insert Threads: 5/16-18;