Fits Seacock: 1250 and 1500. Diameter 5.38″. Insert threads: 5/16″-18. Press threaded brass inserts into the hull side of the backing block into the holes that match the seacock flange. Rigid 1/2″ thickness provides secure installation but is flexible enough to conform to minor hull curvature. Will not swell or absorb moisture. Made from an environmentally friendly PVC foam specially formulated for marine applications.