USE OF "O" RINGS IN MINIATURE STEAM ENGINES
Pistons and other close fitting components derive their ability to withstand the high operating pressures encountered by providing a minimal amount of clearance between components and a very well polished or honed bearing surfaces. You just can not provide a perfect seal and free moving condition with metal to metal conditions. In cases where a close fit is not enough, we can resort to a number of types of seals. Traditionally, pistons rely on either a very close running fit of less than a few tenths of a thousands of an inch for low compression and or small displacement situation. When higher displacements or compression pressures have to be dealt with, we employ piston rigs and round seals to prevent pressure loss. In model engine designs, pistons will either have piston rings made of cast iron, graphite cord rings packed in a groove, or nothing at all. The newest technique in steam proof seals is the use of special "O" rings. Not just the common rubber type used in plumbing work, but ones made out a special material with the proprietary name of " Vitron ". Common rubber rings would soon deteriorate in the constant presence of petroleum derivatives like oils and other lubricants. " Vitron " is a synthetic that has a great resistance to all of these substances, besides being resistant to the high heat produced in an operating steam situation. To further decrease the effects of wear, you should lubricate the surfaces that will be making contact with the "O" rings with low viscosity silicone grease which is compatible with rubber or Vitron "O" rings. These rings can provide a perfect seal in valve housings, pistons and in the piston and valve rod glands.
I am currently in the process of building a single cylinder dual action engine that will incorporate the use of these "O". Two rings will provide a seal for the upper and lower ends of the piston in the cylinder and a smaller one will serve to seal the piston rod in the lower gland. If you do not provide a good method of sealing the lower end of the cylinder, the piston will not have little to no power on the up stroke because the pressure will leak out around the piston rod and the gland bore. The top of the cylinder does not have an opening for steam to leak out. The "O" ring will provide the seal at the bottom end.
How much smaller than the cylinder bore should the piston be made and how deep should the "O" ring grooves be? The normal piston/cylinder fit is in the tenth thousands of an inch but the use of "O" rings changes that. You will have to turn the piston to an undersize condition of about 4-5 thousands so only the rings themselves come in contact with the cylinder bore wall. You also need to plan for about 3-5 thousands worth of compression of the rings in the bore to provide a good seal but not so tight that it's difficult to turn the engine over. How much, depends on the diameter of the bore of the cylinder. The larger the piston diameter, the more compression you will have to provide. The amount is not a lot, but it will be in the range of three to ten thousands of an inch. The groove dimensions are also very important as they must be just deep and wide enough to allow a tight, slightly stretched fit of the "O" rings. The new diameter created by the "O" rings has as I previously mentioned, also must allow for a slight compression fit between the "O" rings and the cylinder bore. Most people will rely on a sense of feel rather than actually measuring the diameters. The actual seal in a ringless situation is created by the oil film between the two. With "O" rings you can achieve an actual physical seal between the running surfaces. Much better situation here. The only trick here is to carefully tune the amount of ring compression to the point where you are providing a perfect seal without creating excessive friction and drag which will counteract the extra power you were after in the first place. I have a method to insure that I achieve the correct amount of seal and "O" ring compression. Instead of boring out the cylinder and them fitting the piston to it, I make the piston about .010" undersize, cut the ring groves and install the ring. I take reading across the new diameter created by the "O" rings, making sure not the compress the rings and bore out the cylinder to the final diameter. As I near the correct bore size, Insert the piston in it and check to see how free it can slide in and out. I want to reach a point where there is just a bit of drag. This will disappear when the silicone grease is applied. Another big advantage of "O" rings is that your engine's power train will not experience any of the normal metal to metal wear normally seen in ringless situations due to the fact that only the "O" ring's surface is contacting metal. What will happen is that eventually you'll have to replace them with new ring set if you experience a loss or power and detect a leak.
Compared to the traditional method for achieving a leak proof seal, "O" rings are not only much more convenient to use, but they prevent the abrasive metal to metal contact so no wear occurs. Besides they cost only about 50 to 90 cents a piece.
Ps. I have since finished a 7/8" bore - 7/8" stroke single cylinder, double action, horizontal engine with double flywheels that incorporates the use of "O" rings on the piston and piston rod gland as seals. It also features a double acting cylindrical slide valve rather than the traditional rectangular one. After some initial valve positioning and timing, the engine was connected to an air source and it immediately ran perfectly and was able to be slowed to around 40-50 rpm, running in perfect smoothness. Pressure required to run the engine was minimal, able to run on lung power. I will from now on incorporate this method of sealing and engine.