This series of engines are usually to be found on a variety of industrial applications such as Generators, Pumps, Compressors, Winches, Diamond Cutters, Floor Cleaners, Stump Grinders etc.
They have an opposed twin cylinder arrangement with the air filter sat on top between the cylinders.
They are side valve with the spark plugs set near the centre of the flat cylinder heads.
The fuel tank will usually be remote (fitted away for the engine).
If there is a pull start, this will be on the side, with the crankshaft emerging horizontally on the opposite side. Electric start only versions have a flat mesh covering the flywheel on the side. There is also an option for a drive shaft extension on the flywheel side.
The Model, Type and Code numbers are either stamped into the pressed steel engine cowling at one end or along one of the sides. Some have them on a small aluminium plate rivited to the top of the cowling. The Model number will begin either 40 or 42.