Groco Seacock Backing Block Use 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 12 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 W