Answering Mr. Brook's question yesterday about Standard flap valves.

The valve described was a rectangle of heavy, but smooth leather, waxed
or varnished on the outside to make it less porous.  It should be about
as wide as the distance between the tack holes plus 1/2", and long
enough to extend beyond the other end of the air holes about as far as
the tacked end is from the nearest air hole.

Glued to the free end of the flap, there was a wooden stick, about 3/8"
square and as long as the width of the flap.  This stick has a small
hole drilled part way through the stick, not on the top of the stick,
but on its side, facing the tacked end of the strip.

The tip of the spring goes into this hole.  The spring should be bent
so that it presses the flap gently against the bellows board, and at
the same time, pulls the flap firmly downwards, away from the tacked
end.

Here are two tips that will make the final result more successful:

1.  Sandpaper the finish off the leather surface, for about 1/2" at one
end, so the glue will stick.  Glue the stick onto this end, with the
flap laying on a flat surface, and let it fully dry.  Then reinforce
this joint with three small tacks.  If you try to tack the flap to the
stick while the glue is still wet, the leather will shift a little,
producing wrinkles or distortion in the leather.  The finished flap
must be perfectly flat at both ends to seal properly.

2.  For the same reason, don't try to just tack the fixed end of the
flap to the board.  First punch three small holes in the center and the
two corners of the fixed end of the flap.  They have to be real holes,
about 1/16" diameter, not just prick marks.  Then put the flap in
place, and adjust the spring so that the flap covers the air holes
properly when the tacked end of the strip is pulled up to where it
belongs.

The spring should be tight enough so that the flap lies flat against
the board, about an inch below where it belongs, when you let go of it.
Take a large tack, put its point through the center hole in the flap,
and pull the flap up to its final position.  Pick a location near the
old center tack mark (but don't reuse the old tack hole) and drive the
tack almost, but not quite, tight.

Now you can check that everything is properly aligned.  Then take two
more big tacks, and pull the corners of the flap just tight enough so
that the tacked end is also flat against the bellows board.  Only when
you are sure that the flap lies perfectly flat along its whole length,
and the loose end slides freely as the flap is lifted slightly at the
middle, can all three tacks be driven tight.  Sometimes this takes a
couple of tries, but it is worth the effort to make sure the flap lies
perfectly flat against the board, with no wrinkles.

Richard Vance