MC,

I am installing an outdoor shower on a concrete wall. For this reason
the shower valve needs to be flushed with the concrete wall surface.
I have chiseled from the inside to hollow out the concrete block wall
by eliminating the web in the middle. This space is right big enough
for the valve which is about 5.5" in diameter. Into the valve I need
to thread in two supply lines and 1 outflow line. The supply lines
come up along the inside wall and have to somehow make their way to
the inlets of that valve on both sides. The wall cavity I have
choseled out so far will be barely wide enough for the valve itself
plus two elbows one on each side will give it a snug fit.
I can chisel out more space with my hammer drill but the problem is
the valve is on the outside of the wall, I am hollowing out from the
inside (the outside is already neatly cut to the shape of the valve),
In order for me to make additional room on either side my chiseling
has to be like 7.5" deep. It is difficult to do so unless I want to
break it all the way to the middle web of the blocks on either side.

OK, that makes sense...

Just a thought, rather than trying to fit the valve and both elbows into a
single block space, could you drill a small hole into the next block space
and run a short stub through the web and put an elbow in the other block
space? For that matter, you could run stubs to each adjoining web so each
elbow is in it's own block space. I don't know if that would be any easier,
but it may give you a little more working room.

Of course, if you're in a cold climate, you'll want to insulate the block
spaces when you're done.

Good luck,

Anthony