< Piston Ring Modification >
Now I return to my workshop for a short while.
This is report of the work that I already
done before the trip.
I didn't satisfy my piston valves because
they were too tight. Piston ring's tension
is too high. So I decided to remake them.
Kozo Hiraoka's book "The PENNSYLVANIA
A4 SWITCHER" shows how to make ideal
piston rings. I referred the method for the
modification.
I don't want to decrease "gap"
because it was already enough small (0.6
mm). The section of ring was 2 x 2 mm. I
decided to decrease ring thickness to 1 mm.
As same as before, turn I.D and O.D of a
phosphor bronze bar, and part off one by
one. O.D is left a little larger than final
diameter. After each parting off, I cleaned
the face of the blank with a fine cut of
a tool.
Another side is also cleaned with a jig,
grinding with emery cloth on a flat surface.
Cut a gap. This time I cut it diagonal with
the angle vise. In Kozo's book, "step"
cutting is introduced as an ideal method,
but 1 mm is too thin to do that.
This is the point! The ring is squeezed by
a fine steel wire. I employed 0.3 mm stainless
steel wire. The wire is tightened until the
gap closed. Then the ring is put onto a mandrel
in three-jaw. A top cap is bolted onto the
mandrel and they hold the ring tight so that
the gap never opens after release the wire.
The mandrel diameter is slightly smaller
than the ring's I.D. So the ring is fixed
eccentrically.
Finish O.D with the mandrel. In order to
prevent burr, the mandrel is also cut with
the ring to the final diameter. From the
second, the rings are finished until a tool
touch to the mandrel.
It is a finished piston ring. With this method,
the piston ring becomes true circle when
it closes. Note the width is different between
the gap and the opposite side. This is result
of the eccentric chucking. This profile can
provide constant ring pressure around the
circle.
The guide rings are also remade due to the
piston ring thickness change. They are made
from phosphor bronze bar too.
Piston valves are reassembled with the rings.
With them, the movement became much better
than before.