PTFE piston rings

Just bored the cylinders for my latest loco (gunmetal). There are some pinholes at one end of both castings. Can't send them back as I bought them second hand, anyway I'm not sure it's that bad. I'd like to try PTFE rings, backed up with an O-ring to give the out spring. I'm sure this has been done, but I can't find any reference in previous threads. Anyone got actual experience?

I tried this a few years back with bad results. The rings expand after machining and even more so when they get hot . Piston siezed in bore, engine thrown under bench...still there somewhere!

Hi Duncan,

I am very old fashioned so would fit graphite square asbestos packing to your gunmetal cylinders properly fitted - which few appear to know how to do properly.

However a friend of mine has recently done extensive experiments with PTFE on piston valves and pistons. I will email you a few links. They take things a bit beyond current thinking.

Cheers,

Julian

I've used them on several 2½" gauge locos with excellent results. Also used PTFE on piston valve heads with equally good results. With these smaller bores though I haven't used backing O rings and just relied on the plain PTFE rings on their own. I fit two thin rings side by side with the gaps 180° apart to prevent leakage. It is essential to allow clearance for expansion when the rings get hot.

I haven't done it yet but I've thought of using graphite filled PTFE for the rings as this material is harder and springier than virgin PTFE and may not need the backing O rings in the larger sizes.

John

Is the shared wisdom to include a bell-mouth at the end of the cylinder to assist with insertion of the rings à la full size practice?