Steam Page 12. Engine parts 1.

For gunmetal cylinders, use soft packing, and for cast iron cylinders,  piston rings.

The size of piston rings can be calculated from formulas by Tubal Cain and Prof. Chaddock. However, the following have been found to  give satisfactory results for general use.

The width of piston rings should be between 1/20 and 1/30 x piston diameter. The modern tendency is to use 1/25 to 1/28.

The "land" between rings is usually the same as the piston ring width.

The depth of a piston ring will decide its stiffness. It is a mistake to make rings too stiff in steam engines as this only increases wear and friction.

A good starting point is to make the depth the same as the width (so that the ring is square in section).

The diameter of a ring (before splitting) should be Bore + (0.001" per inch of bore).

Piston ring material.
With the larger engines it may be possible to find commercial rings that are the required size. Those from motor cycles, garden machinery and small industrial engines are potential sources.
Many of the model engineering stockists keep cast iron in diameters which are suitable for models.
A useful source of cast iron for piston rings is the counterweights used in sash windows, although these are very variable in quality.

Making piston rings.

The actual turning of the ring is basic lathe work, but sometimes makers have problems with splitting them.

The easiest way is to cut them with a small slitting saw or jewellers saw.
With the small sizes, with care, one can file a small nick and then snap them between the fingers.

A safer method is to grip the ring in a vice and give it a sharp blow with a sharp cold chisel, as close to the vice jaws as possible.  (SEE RIGHT.)

The method of splitting rings by forcing them on to a tapered bar often results in multiple breaks and a pile of segments.
(At least, it does when I try it.)

When making piston rings, always make a few spares, in case of breakage or if needed for future overhauls.
(The one sure time when a ring will break is when there is no spare available!)

If buying them, it is worth buying one or more spares at the same time. You might break one when fitting it, or they may not be readily available if you need one in the future.

To adjust the ring gap, slide the ring into the top of the bore and file the ends to give a gap of 1 to 3 thou per inch of diameter. If it is a pegged ring, the gap should be the size of the peg plus 3 to 6 thou  per inch of bore.
The ends of the ring should be checked to ensure that there are no burrs which could hold it above its seating or scratch the bore.

When fitting rings there is less chance of them breaking if they are slightly warm. A few minutes in nearly-boiling water is sufficient.

The groove in the piston should be slightly (say, 5 thou) deeper than the thickness of the ring, and the width should be the width of the ring plus 2 to 5 thou.


Soft packing is the traditional material for steam engines. Originally this was either asbestos string, or hemp, usually coated with a variety of  pastes including tallow.

Today, there are proprietary packings which are readily available in a variety of sizes and shapes.
They work well, and can last for many years. There are numerous examples of soft packing lasting for over 25 years.
Whilst they could be used with any piston and cylinder material, it is usual to find soft packing restricted to use with bronze cylinders.

"O" Rings,.

O rings have been used in many parts of models, and provided they are fitted correctly,  work very well.
The most common mistake is to make them too tight.
The ring manufacturers issue data sheets showing sizes of groove or recess required, and as a general rule these should be followed, although in some cases they may result in a ring which is slightly too tight.
For most steam purposes, rings should just, and only just touch the shaft / housing, and a surprising small amount of contact is needed to effect a seal.

They are not suitable where they pass over sharp edges, for example, in piston valves.


The diameter of the boss of the piston should be between 1.5 and 1.75 times the piston rod diameter, and the length of the boss between 1 and 1.25 times the piston rod diameter.

For coned pistons, the cone should be between 1 : 2.5 and
1: 3.3.

With larger diameter pistons, once the thickness of the boss and of the rim has been calculated, there can be some
advantage in reducing the thickness of the piston between the boss and the rim to reduce weight.

If the piston is fastened to the piston-rod by a taper, the taper should be 1 in 5, or 1 in 6.
A frequent practice in models is to make the taper such that the small end is equal to the o/d of the thread one or two sizes down from the diameter of the piston rod .( E.G., 5/8 rod, 9/16 or " thread on end.)
Too shallow a taper will make it difficult to remove the piston.

Piston rod gland.

Piston rod glands should be just tight enough to prevent leakage but not so tight that they add friction
Finger-tight plus 1/6 to 1/8 turn should be enough to make a seal.

Piston Rods, Cranks and Valves on the next page