For what it's worth, the Ken Swan drawings for my Jessie loco show the piston of the same nominal diameter as the bore. There's no specific clearance drawn. This isn't like Ken - he tends to be very thorough, so let's suppose it's intended to be a sliding fit. The piston is 9/16" thick in a 1.5" bore, with a 1/4in groove for two 1/8" rings. But the piston thickness is related to the cylinder length and the throw of the crank. For Jessie the cylinder length is 3.5in. Add two gaskets of 1/64" each. Take off the stroke of 2.75in, take off the inset of cylinder covers - twice 1/16", take off a clearance at each end - twice 3/64" and you end up with the 9/16" thickness. I don't know if locos have more clearance than stationary engines, because they have to cope with the crank moving with the suspension.
Since the example I've given is for a loco, it's got to start againt a load of passengers. If your engine is just decorative, it can get away with less agressive rings . Perhaps a single Clupet ring would do, and the 3/8" piston would be fine.
Smaller spigots as noted above give you some extra, too. We need a comment from an exising stationary engine builder.
Post by ettingtonliam on Nov 25, 2012 13:43:55 GMT
Samson Are there any steam ports in the cylinder, either drilled or cast in? The Stuart Turner No. 1 engine has a 2" bore and stroke, and it has a cylinder 2 7/8" long. The piston really shouldn't cover the ports at the end of the stroke, so if they are cast in already, you need to measure the distance between ports in the cylinder. I wouldn't like to see a piston less than 1/2" thick, and you could get away with 1 ring, the engine won't be doing much work will it? Andrew Smith in his book on building the No.1 recommends boring the cylinder until the piston ring fits in with a small gap between the ends of the ring, and then machine the piston to as close a sliding fit as possible with the best surface finish. He also says that if you overbore the cylinder and the gap is a bit big, don't worry about it, and even if you really overdo it, forget about rings and use graphited yarn packing. The important thing is an excellent surface finish and a nice sliding fit. If I was finishing off this engine, I think I'd be tempted to do another crank disc with a smaller stroke, say 1 3/4, or even 1 5/8, and get a decent thickness of piston.