The last 6 or so IC engines that I have done I went with a single Viton ring in each, all run OK, make good compression and will turn over with very little friction.

Yes, for a 16mm bore I would go with a metric 2.4mm cross section metric ring of 11.6mm ID. Groove 2.6mm wide x 2.3mm deep

Seems to be a common practice these days to run viton rings in model IC engines, which would seem to defy common sense yet seems to work.

Making cast iron rings is quite a bit of work by comparison to machine them oversize, cut them, squeeze them down to size and then machine to final roundness and size and to heat treat them. Never seen multiple cast iron rings in one groove. I don't think there would be any advantage to it. Multiple rings in multiple grooves has the advantage of acting as a multi-step labyrinth seal so the second ring/groove only has to control the blowby from the first, which means blowby past it is minimal. Then the third ring — in full size engines but not used in models often —  is usually an oil control ring to scrape away excess oil coming up from the crankcase below. If all that can be done with a single viton o-ring it sure makes life simpler.

Edited By Hopper on 09/03/2021 11:11:43

NDIY, like Brian I only give my engines a short run for the fun of it so maybe 10mins long a few times when new and then just the odd run at a show or to let others see them running. Graham Corey of Alyn Foundry first put me onto them for IC engines as he had good results with them, He was very much into going to engine ralleys attending quite a few each year and would have his engines running all day so life if long. These would be iron bores, drip feed oiler and reasonably slow reving.

As said ringless is also possible with the smaller engines, I've done it on aero engines and also run some of my others without fitting the rings.

Modern engines can survive very well using two gas backed compression rings (With minimal gap ) and one oil control ring. But the dimensions and surface finishes are closely controlled for ring, groove, and wall pressure as well as the surface finish of the bore.

Too large a ring gap increases blowby, and damages oil control (Compression rings do influence oil control! )

Piston / bore clearance and piston attitude also affect noise, blowby and oil control.

Older engines used lower wall pressures, bigger gaps and ring / groove clearances, and rougher bore conditions.. Hence the need for three compression rings and two oil control rings, to deliver inferior oil control.

(With the need for careful running in, to provide oil control and blow by inferior to a modern engine.)

For an IC engine, C I would be my material of choice. A top ring of Viton could be subjected to quite high temperature if the engine is run fast under any sort of load.. Possibly OK for a lightly loaded engine run for short periods at relatively low speed, but less likely to survive prolonged hard work.

Howard