[2] In 1918, ABC made a motorcycle with a 400 cc flat-twin engine mounted with its cylinders across the frame, several years before BMW adapted the design.

They were both at Brooklands and in 1912 a Sopwith with an ABC engine flown by Harry Hawker had won the Michelin Endurance Prize.

In 1919 they jointly exhibited the Sopwith 390 cc horizontally opposed twin-cylinder overhead valve (OHV) machine at the annual Motor Cycle show.

It aroused a lot of interest with innovative front and rear leaf springs and "expanding" brakes, wet sump lubrication, and a four-speed gearbox.

[6] ABC FIREFLY 250cc Only one prototype was made in 1916, the original engine and gearbox where found and used to make an exact copy.

See here; https://www.youtube.com/watch?v=CaFHanJ4FD0 Produced between 1919 and 1925, the ABC 400 had a 398 cc horizontally opposed twin-cylinder overhead-valve four-stroke engine, four-speed gearbox with an H-gate and was fitted with an advanced (for the time) carburettor from Claudel-Hobson to give a top speed of 70 mph (110 km/h).

The shift from producing aircraft to making motorcycles was more difficult than ABC expected, and their costs – and prices – were higher than the new competitors emerging after 1920.