The Unit Steam Engine, commonly abbreviated to USE, is supplied as a single cylinder unit but is designed to be built up into multi-cylinder engines. In addition to a single unit, I also have four mounted as a four cylinder engine. 2 and 3 cylinder configurations are also common. As a single cylinder, the engine is fairly conventional but with design details intended to facilitate coupling together. They can be bought either ready built or as a kit of machined or unmachined parts.
The base is sized so that the units are at the correct spacing when the bases are butted together. The base has two pips cast into one end which fit into indentations in the other end.
The steam feed and exhaust pipes are parallel to the engine axis and have openings on both sides. This allows the pipes to be connected together in a daisy chain manner. The pipes at one end of the chain have to be plugged, usually with removable screws to enable later extension.
The flywheel has a hole in the outer face into which can be inserted the extra long crank pin of an adjacent unit. The flywheel is fixed to the crankshaft with a grub screw so that the position of the hole can be adjusted to obtain the correct phasing depending on the configuration.
The units are normally coupled together in an in-line configuration, although a "V" configuration is possible with added parts and some modification of the units.
The cylinder itself is a single acting oscillating cylinder, turned from square section brass. It is 5/16" bore x 3/4" stroke. The engine frame is cast in what appears to be mazac.
As acquired, most of the engines were working but were a bit stiff with rough spots in the cylinders. Running in on compressed air for several minutes seems to have improved the feel slightly so I hope they will improve every time they are run.
The video below shows a four cylinder configuration.