Hi Alan. I'm no expert but my thoughts......The dove-tail side should be
on the inside of the groove, with the "O" ring " just small enough to snap in
without really stretching it.
The gland ( groove ) should be wider than the "O" ring to allow for
"squeeze" so if you look at the charts in the Parker book it states the depth
and width for each size diameter of "O" ring.
They specify metal to metal contact under pressure and gives a tolerance
for fit. Pay attention to this. A little variation can be expected across the
two planes, but not much. The closer to " perfect " the better.
When the pressure is applied and the "O" ring squeezes into the gap
between hatch and hatch land, it should be a VERY small crack. The "O" ring
will actually squeeze into a rectangular shape to match the gland. The water
gets into the groove and pushes the "O" ring up against the crack between the
seat ( hatch land ) and the hatch groove ( gland ) so it's trying to force the
rubber "O" ring into that tiny crack. There needs to be enough extra room in
the groove to allow for deformation of the rubber "O" ring.
I seem to remember a drawing of that in the Parker book.
Upon first submerging, there will probably be a little leakage as there's
little pressure on the hatch.
Because there's a lot of square inches on the hatch surface, and the
pressure is applied across the entire surface, it seals up quickly. The hatch
"dogs" are used to minimize that initial leakage.
As far as the tolerance between the two surfaces, I've seen some pretty
"rough" paint jobs on the two surfaces and they still seal up fine.
Read in the Parker book about the "scratches" when the groove is
machined. Radial scratches are bad, circumference ones not so bad. Either
way you want both surfaces to be pretty smooth.
A good place to get economical "O" rings is a mail order company called
McMaster-Carr.
Buy a couple as they don't last forever and are easy to switch out if one
gets damaged.
Frank D.