and tensioning the ring
After cutting off and preparing the piston ring, we now must give it some spring tension and expand it, so that it applies pressure outwards against the cylinder wall when fitted, and helps make an air tight seal.
This is the one area which puts many people off of attempting to make piston rings.
There are endless complicated processes and write ups on how this should be done, using fancy jigs, temperature controlled furnaces, controlled cooling, cold forming, exacting annealing etc - you name it.
The fact is, cast iron is not significantly affected by heat, and unless chilled (which we don't want to do in this application) has quite a stable Rockwell hardness factor - so all this mumbo jumbo is a bit over the top.
I just use a simple method suggested by a model engineer on the web, who seemed to be regarded as a bit of a crack pot by his fellow club members. But it works extremely well.
All you do is spread the ring gap to the degree of tension/expansion you want by pinching it onto a suitably sized (diameter) metal rod. This varies slightly between applications, and is about 15% of the external diameter of the piston ring.
Make sure the ring ends gripping the rod are parallel and free of burrs.
The rod is then placed on some red house bricks with the ring hanging downwards - as seen below.
Strong heat is then uniformly applied to the ring until it glows (or gets near to) a cherry red colour.
The ring will then fall from the rod under it's own weight.
Allow to air cool.
The ring will hold the degree of set applied by the diameter of the metal rod, and have excellent spring tension.
Here's a video showing how I do it.
You can even reset used rings from overheated engines with this method - but they will need to be run in again.
of ring gap finish
Standard parallel 90 degree type below - particularly for modern thin section rings.
Angled overlapping type below - close tolerance with minimal blow by for old style deep section rings.
Notched type to mate with an anti-rotation peg below - primarily for two stroke application.
NOTE: Some modern two strokes engines do NOT incorporate an anti-rotation peg, or have a square peg slightly less than the width of the ring gap. In this case the ends of the ring can be machined parallel.
Setting the ring gap is dealt with on the next page.