Build - Piston Rings

Piston Rings for the Webster Engine

[Jan. 31, 2017]

As the title indicates, this page is about making piston rings for the Webster engine (see build here). Since making piston rings is a moderately involved process I decided to dedicate a web page to this effort.

References

For a list of links on making piston rings, go to this link and scroll down to "piston rings." There are a number of different methods for making piston rings; after reading through the various articles, I decided to go with this basic method:

1. Turn and bore a piece of fine grained cast iron to the required dimensions
2. Cut off rings with a cut off tool (including making extras)
3. "Snap" rings to create gap
4. With a purpose made jig, open and heat rings to "set" to expanded size
5. With a purpose made jig and abrasive paper, lap rings to required thickness

Relevant Dimensions

I measured the parts which I had previously made for the Webster engine to get the following dimensions:

Piston O.D. = 0.872"

Piston groove O.D. = 0.773"

Cylinder I.D. = 0.876"

1. Turning the Rings

A piece of cylindrical cast iron is chucked in the lath and bored to an I.D. of 0.774" (the O.D. of the ring groove plus 0.004").

The O.D. of the cast iron is then turned to an outside diameter of 0.877" (the cylinder I.D. plus 0.001"), and polished to a close fit in the cylinder.

480 grit abrasive cloth (shown sitting on the headstock) is used to polish the outer diameter to a close fit. During polishing, the outer diameter is checked in three places with a micrometer to ensure even polishing along the length.

After about 10 seconds of polishing, the piece is wiped clean and the cylinder is used to check for fit. Micrometer readings are used to adjust the polishing to make sure it is uniform along the length. With 480 grit abrasive, I found that 10 seconds of polishing removes about 0.0005" (1/2 thou) of diameter.

2. Cut Off Rings

The rings are cut off with a parting tool (in this case the parting tool had a width of 0.040"). The rings are cut off to the groove width +0.001" (to allow some extra width for final lapping).

Note the very thin ring which parted off just after the ring was parted. A rod chucked up in the tailstock is used to catch the parted rings.

To ensure a good smooth cut, the parting tool was honed with a diamond file prior to use.

Here are all of the (not yet gapped) rings. I ended up with seven rings. Five extra rings is probably overkill, but as this is the first time I have made piston rings, I wanted to have plenty of spares,

The rings were deburred on both the inner and outer diameters using a Cratex polishing point on a Dremel tool. To do this, the ring was simply hand held and the Dremel tool was applied lightly at a 45 degree angle to the inner and outer edges on both sides.

Here are both a deburred ring (on the left), and a not yet deburred ring (on the right). Click on the picture to enlarge and see more clearly. It's actually easier to feel rather than see the presence of a burr.

3. Snap Rings to Create Gap

To snap a gap in a ring, the ring was held in a small vise as shown. The vertical edge of the vise was lined up with the center point of the ring (i.e., half the diameter). I found I had best results when the ring was clamped fairly firmly.

Smooth jawed pliers were used to hold the ring and apply bending pressure until the ring snapped. Unlike the angle shown in the picture, I had best results when the pliers were held at a slight upward angle (i.e., plier handles above the snap point).

I tried nicking the ring at the snap point with a fine file, and also scribing a line at the snap point with a carbide scriber but found that neither of these worked any better than doing nothing at all along these lines. Just placing the ring in the vise and snapping it worked well - all seven rings were snapped without breaking the ring.

To the left is a snapped ring - I've marked the snap point with red lines on either side. The gap is very hard to see, but if you click on the picture to enlarge it, you should be able to make it out.