A well made flat four side valve engine to Len Mason's "Mastiff" design. Appears never to have been run, although turns over freely, with compression.
The design is 3/4 bore x 7/8 stroke, 25cc displacement with a 2:1 skew-driven shaft at the rear driving oil pump and distributor. Cylinders and heads are jacketed for water-cooling, fuel is supplied via a single down draught carburettor.
The builder has thoughtfully stamped the engine with cylinder numbers, points gap on the oil filler cap, timing marks on the flywheel and firing order on top of the engine (1-3-2-4) - along with an exhortation to use Castrol oil.
Height from base to top of engine 8 1/2 inches