Piston rings

Hi all, I have just about completed the cylinders for my 5" gauge BR Standard Class 2 to Don Young's plans.

The pistons are designed to have 1/4" square graphite packing as the piston seal / ring. Could an Viton O ring be used for the seal ?

Peter

I’m not a model maker, but if the material is made for that appication. I see no reason for not using it. Might need a different sized groove...

Are you using gunmetal cylinders or cast iron?

The cylinders are cast iron.

Never heard of anyone using o-rings in cast iron, but perhaps I'm just lacking knowledge. Slightest bit of rust would rip the ring. Why not make proper cast iron piston rings? Not terribly difficult, I've done it and I'm not one of the demi-gods of the machining world. Proprietary ones work but tend to be too thick and stiff.

Peter

The o-ring will wear a flat face very quickly. It will work for several hours until it does.

Ideally you want to fit cast iron split rings and I appreciate these can be tricky to make. I have just made a set and If you want help then message me. If your bores are accurate to diameter then you can buy rings from someone like Reeves, but they are about £13 each.

An alternative is split PTFE rings riding on top of o-rings. But this is as tricky to make as cast iron rings IMHO.

If you pack with a PTFE/graphite square material it will work fine for a few years, but be prepared to replace it one day. Not the end of the world and the quickest solution.

Norm

Why not use what is specified, worst price I've seen was £4 a metre.

Here you go.

Bore to size, surface to size, part off using a toolpost grinder (fitted with a thin cutting disc), clean up or thickness on a flat abrasive suface, split, suspend on a suitable spreader, heat evenly until it falls off. Clean up ring ends and set ring gap to size. Job done.

Edited By not done it yet on 14/01/2019 15:11:04

If you've already made your pistons and cut the grooves then go with graphited packing but search for comments about graphite yarn/packing as doubts seem to exist about what you get, from where, these days.
Be careful how tight you pack it into the grooves; too tight and the wall pressure will be far too high especially before eveything is up to temp.

IMHO it is well worthwhile reading Tubal Cain's section on piston rings in The Model Engineer's Handbook (summary of his series in M.E. 1994). If you decide to make your own CI rings then you will need to make a sizing sleeve on a machining fixture for final skimming otherwise it is highly unlikely your rings will be concentric with the bore (with apologies to NDIY).