[casey1999]

Groco has backing blocks similar to Mainesail's no through bolt method. They don't state what material they are, I guess fiberglass.

From Groco's catalog:

A few months back I called Groco and asked what material their backing plate was made of. The tech support did not know exactly, but said the material is like a man made plastic plywood. It is not solid and the literature states it can form to non flat surfaces. I am sure you cannot drill and tap the Groco backing plates. The backing plates would only be of use if you plan to through bolt a seacock or Groco adaptor plate. These cannot be used like Maine sail drill and taps the G10 and epoxies to the hull.

 

[casey1999]

I couldn't find any place that sells them. I thought I could save a step and save having to buy a big hole saw if I could buy a pre-shaped backing plate, but although they're in the catalog I couldn't find any website selling them.

Yes I was hoping the same thing. If you want them, just go to the Groco website and look up Groco distributors, they can special order them for you. In any case, they do not look like they would work. In fact, I am really not sure how they can be used. It looks like Groco would expect the installer to use the thru hull itself as the sole method of connecting the seacock to the hull. This backing plate would just distribute to loads on the hull. This would be no different than sticking a piece of plywood under the seacock (or adapter fitting). With the thread inserts, it looks as if Groco is not expecting the installer to through bolt the seacock/adapter plate.