Cast Iron Piston Rings

Some years ago I made cast iron piston rings, however, in the nearly 40 years since I have completely forgotten how. So, I’m reading lots of forums, references and articles on making cast iron piston rings. Whilst doing this I thought it was worth writing an article on the subject. I have listed references where appropriate for further reading.

Piston Ring Size

What should the cross-section of the ring be? Remember that if the radial thickness is too great it will be difficult to open the rings enough to fit them and the forces on the cylinder bore will be greater. Thus increasing frictional losses.

  • Diameter
    • Machine the diameter to match the bore of the cylinder
  • Radial thickness
    • Full size IC engines use bore divided by 22 to calculate radial thickness
    • Model steam engine rings use bore 0.04 to 0.045x the cylinder bore
  • Width
    • This is not as critical, 0.1x the bore is a good starting point
    • The width of the ring needs to be around 0.025mm (0.001″) less than the width of the groove in the piston
  • Butt joint gap
    • The gap between the ends of the ring when installed
  • Spring open gap
    • 0.15x the bore of the cylinder

Heat Treatment

Lots of discussion around temperature, time and finally quenching.

  • Temperature: 595°C (1100°F)
  • Time: 1 hour
  • Quench or air quench
  • Ensure even temperature around the ring
    • heat in a kiln
    • clamp ring in larger piece of cast iron to distribute blow torch heat

If you have a different method or different sizes/temperatures then do drop me a line as I would like to collect and document all of the different methods.

References

  1. IC Engine Piston Rings – Forum post on Model Engineer
  2. Heat Treat Rings – Forum post on HMEM
  3. Making Piston Rings – classic method originally written by George Trimble and published in Strictly IC

Leave a comment