Once that was mastered, in around 1901 John Prestwich designed and built his first engine and 1902 his company moved into the production of motorcycle engines. From 1904 until 1908 the company produced complete motorcycles, with a Jap Engine fitted into a BSA frame with sprung forks and a vertically mounted 3.5hp overhead valve engine with a single push-rod opening both valves. JAP was the first company in the UK to design and build an overhead valve V-Twin motorcycle engine. The production of entire motorcycles, however,  was short-lived for JAP but the company continued to hone its engine manufacturing skills and diversified in many directions. For example when WW2 struck, John Prestwich's company produced around 240,000 petrol engines for aircraft and agricultural machinery making a significant contribution to the war effort.

By 1908 the company had moved away from the production of whole motorcycles, and JAP began supplying engines for agricultural machinery, aircraft and other motorcycle manufacturers like Brough Superior, Triumph and Royal Enfield. In 1945 the company's engine production was taken over by Villiers, and went on to supply engines frequently used in speedway and racing bikes with great speed successes right into the 1960s. Variants of the Jap engine, sometimes modified to run as 4 stroke alcohol fuelled engines, were used by enthusiasts for dirt track and speedway racing right into the 1970s.

All in all a great story of innovation and success in the age of combustion engine pioneering.