If I remember correctly, not a given, only one of the three rings was a compression ring. One was to lubricate the cylinder wall, one was to seal the oil in the crankcase from getting into the combustion chamber and one was for compression. The only justification for more compression rings is the chance that one of them would work properly.
Hi Gordon, I guess we come from different parts of the industry, and possibly at different times.
I started in the mid-1960s and I think the oldest engines I re-bored and honed came from the 1930s. At least a few Austin 7 side vale engines, some old diesels (with cast-iron pisotns, rings etc.)...
I have only ever come across a single oil control ring used. But if this is a proprietary oil ring design it may be a pair of steel rings with a spring steel carrier. No use for models anyway.
But I do remember 3 compression ring pistons, and (memory may be wrong...) I think these were old diesels with cast iron pistons, but there may have been some Ali pistons for diesels with 3 comp rings.
The principle of multiple rings is simply to replicate the multi-staging of pressure drop, as with labyrinth seals, Tesla's non-return valve, turbines, etc... to reduce the volume of blow-by gases that pressurise the crankcases. But the principle of mimic is in the number of rings is to reduce friction, accepting the increased blow-by.
If you have a 2-stroke engine using the crankcase as the charging pump, then increased blow-by can be tolerated, as on the next stroke this just means a tad more gas in the cylinder.
I'll reiterate what I remember of the Hepworth and Grandage ring manufacture (as they made millions of rings every year that went into engines in a pre-run-in condition):
1. Machine the ring blank to correct diameter for being "sprung open".
2. Slit, with a gap equal to final gap plus amount of sprung-open gap that will dissappear when compressed.
3. Install in a cylinder of size plus finish grinding diameter, and clamp a pack of rings onto the mandrel.
4. Install mandrel with rings in grinder and finish grinding to size.
5. Install rings on a loose mandrel in a slave bore of size, and "Barrel" finish with finest lapping compound oscillating axially in the bore and rotating 10 to 30 degrees between each oscilation. This is to maintain the perfect circle from grinding, not make an out-of-round into a circle. This simply finishes the surface, so from first start in the factory, the engine has the planned compression and blow-by, and surface finish to avoid the need for running-in. (Modern customers don't know what "Running-in" is all about!).
If good enough for the car industry, it should work for the model... I think?
Perhaps the issue is the sizing of bores as setting gauges, lapping bores, etc...or the amount of spring in small rings? (Is that the reason for annealing?). Maybe the fitting process is distorting the rings from their perfectly made circular shape? Perhaps the annealing is changing the shape of the cast iron (it does!)...?
Try making steel rings from rolling flat wire instead?
Perhaps it is the bore, not the rings?
What does marking blue show the contact points to be?
Maybe I should try making some myself so I can learn to eat humble pie when I am wrong?
Enjoy,
K2
