O ring cylinders for vertical Steam engines

[Reggleston]

Currently building vertical steam engine number 3. First was a scratch built 2x version of one of Rudy K's design having a Delrin cylinder and piston without rings in a close fitting bore that runs fine on CA, 2nd is a bottle style frame engine with a cast iron cylinder, Delrin piston without rings that also runs fine. Both engines have enough torque to require a firm hand grip to stop rotation, 3rd engine I am building is also a vertical frame(stewart) type design also having cast iron bore with a Delrin piston with a 1/8th dia. O ring on the piston. As this is my first attempt at an O ring piston I could use any advise from others who have experienced this type of combination cylinder and piston. Any rule of thumb as to what depth of groove is suitable for the piston with the O ring? First trial O ring fit is with the O ring extending into the cylinder about .005 larger diameter than the piston. Fit in the cylinder seems adequate, question is will that provide enough compression to run the engine on Compressed air?
 When you cut the groove in the piston for an O ring is it advisable to use a rounded radius tool or use a flat tool for a flat groove, or does it matter? How much under bore size should the piston be with an O ring type piston. Any thoughts and experience on O rings and piston clearance would be appreciated.
Thanks.
Bob

[AOG]

I generally undersize my piston by .005 inch when using o-rings. As to depth and width of cut it depends on the size of the o-ring. I use the table in the model engineer’s handbook for the specifics. As to shape, I generally use a modified parting tool to cut the groove. There is some small radius in the corners but it’s basically a square cut.

Tony

[Jasonb]

I've never used delrin or rubber rings, usually aluminium 6082 and Vitron O rings.

For air running allow 0.001" per 1" diameter bore, I don't generally measure this but just go by feel. If made well this will allow you to run the engine slowly with no load on a couple of psi of air WITHOUT the ring, just put the ring in if you want to load the engine.

At the very least hone the bore but preferably lap it.

[crueby]

Definitely use the square bottom cut, it gives the ring some room to 'squish' into when compressing it into the bore - does not take much compression to give a good seal. I also use Viton rings, seem to wear very well (always like to keep a little oil on them too). The tables are a good place to start, but as Jason said I also cut it close, then trial fit and nibble away a little more till it feels like it slides in without too much binding. Not sure how consistant the rings are one to the next, and one manufacturer to the next. Fortunately they are relatively cheap to buy, I get them in packs of 25 or 50 from places like Amazon, they carry the whole range of sizes.