Groco's Flanged Seacocks installation is improved with the use of a backing block between the valve flange and the hull.
The pliancy of the material used in the block helps mate and seal the flat surface of the bottom of the seacock base to hull surfaces with a minor curvature.
Each size of backing block fits two seacock sizes (except the 5" block)
1/2" thickness is rigid enough for a secure installation, but flexible enough to conform to minor hull curves
Will not swell or absorb moisture
Threaded bronze inserts match seacock hole pattern, leaving three additional holes for mounting the block