Tail rods on pistons. There are basically two types of tail rods on pistons. A. Tail rods that pass through a sealing gland in the front cylinder cover, similar to the arrangement in the rear cylinder cover. B. Tail rods that do not pass through a sealing gland in the front cylinder cover, but are kept steam tight by means of a long pressure tight cover over the extended rod. For type A the tail rod does indeed affect the surface area that the steam works against, in the same way that it does at the back of the cylinder. For type B the tail rod will either be hollow or will have some other passage to connect the space inside the long cover and the cylinder itself. This passage allows the steam inside the cylinder to also be acting on the end of the tail rod. Because of this the effective force on the piston is the same with or without the tail rod (ignoring any slight steam pressure drop along the passageway). Tail rods were avoided in most designs as they increase the mass (weight) of the reciprocating piston/piston rod/crosshead assembly and require additional balancing. Attached is a section of a S&D 7F drawing showing the cylinder and tail rod with cover (type B). In this design there is a passage to the end of the cover which can be seen in the photograph of the bearing bushes, steam passes through the ring of small holes. The large threaded hole is for the pressure lubrication fitting. On the 7F the pistons are made of steel and should not touch the cylinder bore, hence the use of tail rods to support the piston. For those who are wondering what the grey pipe is that connects both ends of the cylinder on the drawing, this is a bypass pipe with a valve in the middle. When coasting with steam shut off the valve opens and allows steam/air to pass directly from one end of the cylinder to the other, reducing the pumping action of the pistons. Andy.