Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

The green fluorine rubber rings are widely used in everything from food handling to aircraft hydraulics. It seems their claim to fame is good resistance to oil based products such as gas etc, and moderate heat tolerance. If they’ll seal an aircraft hydraulic pump or actuator, they’re pretty impressive under dynamic conditions. That said, any seal is only as good or as free or restrictive as the dimensional precision in manufacturing, as well as design of the groove.

They’re lower drag than conventional iron rings, that I do know, but also allow more blow-by.

This has become apparent in the recently introduced 32cc inline four ohv gas engine. It was introduced with fluorine rings in chromed liners, and puked oil out the exhaust like a two stroke. There is simply no excuse for that in a wet sump, pressure lubed engine running straight gasoline. I imagine cost and the low drag in the tiny 24mm bores were factors.
Nevertheless, in a couple of months since intro, they’ve created an upgrade kit with stainless liners and conventional two iron ring pistons.

I’m expecting to take delivery of an engine any day, and already have the upgrade set. The rings fit perfectly a sheet of paper just fits in the ring gap with a slight drag.  That’s about spot on for slightly under an inch bore.  Yet they do drag noticeably more than the fluorine rings when assembled in the liner.  That will likely improve some with run in however.

Which begs the question, could the m series pistons be grooved to accept metal rings?  I gotta imagine that’s what the old timers had. A single or double compression ring without an oil scraper ring ought to work fine. If it held blow-by to a minimum, the cost benefit of a sharper and more controllable burn vs the drag might be a good trade.