First off I want to say that I have seen that the fabrication of rings can be a charged subject on this forum and I am no expert so take what I do with a large grain of salt. I was amazed that Brian’s recent discussion of making rings garnered over 30 thousand views over a period of a few months. Wow!
I try to machine to print, but there are so many hours in the fabrication of the sleeves and rings that I want to be able to use a part that might not be perfect. So I make my cylinder sleeves first, then make my rings to match them, then I make my pistons to match the rings.
A recent revelation was Terry’s light inspection technique. I machined an aluminum plug (don’t have any Delrin), painted it black and use a super bright flash light to backlight the rings in the sleeve.
I use George Trimble’s formulas for the dimensions of my rings and the stress relief fixture. Given a 1″ bore, the thickness of the ring should be between .040″ and .045″. I go a little bigger, say .050″ This is rather critical! Too thin and there is not enough wall pressure, too thick and you have too much installation stress (broken ring). The relaxed gap should be .15″ ( I use 4mm – .154″), compressed gap should be .004″. The height of the ring is not too important. Smaller is less friction, say .03125″
I use my CAD program to do my math for me. Below is my calculation for the fixture:
First I take the internal diameter of the split and gapped ring in the cylinder sleeve. That is represented by the circle on the right, the internal diameter of the ring in my case is .900″ Using a half circle allows me to measure the arc length. I determine that the arc length of the internal of the split ring is 2.8234″. This is twice the displayed ring half arc length of 1.4137 minus the gap of .004. These dimensions are placed on the left diagram which represents the heat treat fixture. This gives me a diameter for the fixture of .9455″, a spacing pin diameter of .154″ (I use a 4mm threaded rod) and the position of the gap spacing pin from the center of the fixture of .5038″ (I round to .50″)
Summary of my piston ring fabrication process:
- I put the cast iron slug in a 4 jaw chuck for the max gripping power and rigidity.
- I turn the ring blank down, leaving 300% material, so instead of .050″ thick, I turn down to .150. So, for a 1″ internal cylinder bore, the ID of the blank would be about .9″ and the OD would be about 1.05″
- Pull the blank out of the lathe and Stress Relieve in the heat treat oven – 1000 to 1050 degrees F for an hour and a half.
- Furnace cool to less than 200 degrees F before air cooling the rest of the way down. Usually I just leave it in the oven over night.
- Put back in the 4 jaw chuck centering as well as possible. Turn the ID on the ring blank to final dimension, then turn the OD to final dimension. Finish the OD with emery paper.
- Part the rings off.
- Wet sand (with light oil) on a piece of glass with 800 grit paper to clean up edges and get to proper height.
- Cleave the ring gap. I lightly score the top and bottom of the ring with a fine file, then use a razor blade to cleave the ring. I hold the razor blade so its cutting edge is in the filed groove, then tap the back of the razor blade to cleave the ring.
- Use a fine diamond file to clean up the gap and get it to .004″ in the cylinder sleeve using feeler gauges.
- Use an Arkansas sharpener’s ceramic rod to clean up inside edges.
- Clean the rings and the heat treat fixture with acetone.
- Clamp rings into the heat treat fixture. Clamp means finger tight, everything will expand with the heat.
- Set the heat treat oven to 1050 degrees F and place the loaded ring fixture into the oven.
- When the oven reaches a temp of about 300 to 400 degrees, pull the fixture out of the oven and rub boric acid all over the exterior of the rings. This does a good job of preventing scale, it is easy to see where you miss a spot after treating.
- Stress Relieve the rings in the heat treat oven – 1000 to 1050 degrees F for an hour and a half. (my reference is the study performed by the Naval Research Laboratory on stress relieving cast iron dated 1948)
- Furnace cool to less than 200 degrees F before opening the furnace.
- Remove the fixture and drop into a tin can of boiling water to remove the borax.
- Lightly scotch brite the rings exterior, then remove them from the fixture. Clean the rings as necessary.
- Perform Terry’ light test to determine the quality of the seal of the rings against their cylinder sleeve.
- Measure each ring, note in the log, coat each with light oil, and place into labeled zip lock bags.
Heat Treat oven in use for cast iron stress relief
Heat Treat fixture just out of the Oven. I use a single sacrificial ring to insure the stack is flat against the fixture.
Finished ring installed in its cylinder sleeve with computer screen back light. Ring Gap can be seen at the top of the ring and is .004″.
Light Testing Essentials
Good light test, only the ring gap lets the light through. In a dark closet here, I needed a little ambient light for the photo. You need to twist the cylinder and look straight down the walls to see if there are any light leaks.
What was my biggest lesson learned? Make all of your cylinder sleeves with exactly the same internal diameter! I made one 1.000″ and another 1.006″. I just finished making a set of rings for one cylinder, now I need to repeat the process for the other. It would have been better to make hone both cylinders1.006″ and all the rings the same.