In 1912 Harry Stevens designed a 349cc side-valve engine for John Marston. Its single cylinder has a 75x79mm bore and stroke. The engine powered the first Sunbeam production motor cycle, penned by John Greenwood who had been recently poached from Rover where he had been responsible for a similar model. Greenwood had also worked with JAP. The machine he developed had a two-speed transmission with all-chain drive, and was finished in black enamel with the tank in green. Subsequent models had black tanks with gold pin-striping.
Late in 1912 they entered competition and succeeded - advertised as "The Gentleman's Motorcycle", the Sunbeam name spread far and wide.
Sources: sunbeamsidevalve.com, et al.