Jeffrey Wood is correct in his observation that there should be
a 1/4" gap at the closed end.  However, the crucial measurement, in
my opinion, is not the 1/4" gap at the bottom but that the width of
the opening at the top is exactly 3" from outside edge to outside
edge.  This is what the original was.  If the top is opened exactly 3",
there will automatically be an approximately 1/4" gap at the bottom.

I like the idea of using a spacer at the bottom end, but it should
be such that the opening at the top is exactly 3".  I use a 1-1/2"wide
by 3" long hardwood wedge (I think I made it from an old door stop)
that tapers from 1/2" at the top to about 1/8" at the bottom.  I use
a calipers set open to 3" at the top and slide the wedge in at the
bottom until it is snug.  This usually leaves a 1/4" gap, rarely very
slightly different.

Regarding covering with leather and leaving a 1/6" overlap at the
hinge, this would seem to me to be putting the seam at the worst place
because this is where the most flexing should be.  I believe that the
original factory covering was done as follows:

1. The top end is opened to exactly 3" as stated above.

2. The cloth is slightly more than 3" wide (to allow for placement
and trimming) and 9-1/4" long.

3. Covering process is like any pneumatic, top end first then the
sides.  The 9-1/4" length allows 2-3/4" length down each side.

4. The other end is covered exactly the same way, making sure that the
opening is exactly 3".  Note: I think that it is important for an even
running motor that all six ends open exactly the same amount.

5. The cloth used in the original was 0.010" thick with rubber side to
the inside.

6. When both ends have been covered the 9-1/4" length of the cloth
leaves a gap of about 1" in the middle.  This gap in the original was
covered with a 2" wide strip of relatively thick pouch leather (about
0.020 thick), thus giving a 1/2" overlap on the cloth ends.  This 1"
leather area gives the necessary flexibility and durability around
the hinge area.  To install it glue the leather only to the stationary
board.  When dry fully open one end of the pneumatic and glue only that
half of the leather to the movable board.  When dry open the other end
fully and glue the leather to the movable board.

7. Cloth is trimmed away from the screw holes.  After assembly 3/4"
round cloth patches are glued over the screw holes.

Dave Geissinger