< 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.