GROCO 3/4 to 1-Inch Marine Seacock Backing Block

The GROCO marine seacock backing block gives 3/4-inch and 1-inch seacock installations a purpose-built foundation on the inside of the hull. Its hole pattern is arranged so one block can work with more than one compatible seacock size, which is useful during new installations and refits. Threaded brass inserts are pressed into the hull side of the block at the holes that match the seacock flange. That approach provides a secure mounting point without relying on a moisture-sensitive wood backing pad.

The block is made from a marine-formulated PVC foam that will not swell or absorb water. Its rigid half-inch section supports the seacock flange, yet the material has enough compliance to follow minor hull curvature instead of demanding a perfectly flat interior surface. The non-absorbing construction also makes the part easier to maintain in damp bilge spaces. Because the threaded inserts are positioned to match the selected flange, the installer can build a repeatable bolt pattern before the seacock is finally bedded.

• Fits compatible 3/4-inch and 1-inch seacock flanges
• PVC foam does not swell or absorb moisture
• Threaded brass inserts provide dedicated mounting points
• Conforms to minor hull curvature during installation

Dry-fit the backing block and seacock together before drilling so the valve handle, hose run, and service clearance all work in the intended position. The block should be fully supported against the hull and installed with bedding and fasteners appropriate to the vessel construction. After final assembly, verify that the valve operates freely and that the flange remains evenly supported without rocking. For compatible seacocks and other below-waterline shutoff hardware, browse Seacocks & Ball Valves.