JAP

500 Speedway Twin-Cam -1978

Dishing the dirt

After its victory over the Douglas and Rudge machines in 1931, the JAP single dominated speedway, short track and grass track racing until the arrival of the Czech-built ESO and Jawa machines in the 1960s. By that time, the ]AP company had closed down, but Alec Jackson, a former rider, revived the manufacturing of ]AP speedway and grass-track engines, as well as marketing complete bikes under the Rotrax marque name. In order to improve the performances of the long-stroke (80x99mm) JAP engine, he amended the bore and stroke to 84x90mm.

Old Wine, New Bottle

In 1978, after Jackson's retirement, the firm was run by his former assistant George Greenwood, a highly talented engineer. He retained the bore and stroke but replaced the half-century-old overhead-valve layout with a four-valve, twin overhead-camshaft design.

Outclassed at Last

Though the aluminum casting concealing the camshaft drive suggested that they were geardriven, in fact, a roller-tensioned chain was used. Just fifty of these twin-cam JAP engines were made, and they were particularly popular for the sidecar events, then popular in Germany and the Netherlands. Sadly, the competition proved too strong for the JAP, and - with new engines coming from Jawa and the Weslake company- the shattered Greenwood was forced to cease production and died not long after. However, his son Alec runs a shop in London that still sells spares in the 90Â’s - even for the rare twin-cam.

SPECIFICATIONS

Engine: 499cc (84x90mm) air-cooled singlecylinder four-stroke

Power Rating: 50 hp @ 6600 rpm

Valves: twin overhead-camshafts driven by chain; 4 valves

Fuel System: carburetor

Transmission: transfer box with clutch; direct primary and secondary chain drive

Suspension: short-travel telescopic forks (front); rigid (rear)

Brakes: none

Wheels: 23 inch (front); 19 inch (rear)

Weight: 190 lb

Maximum Speed: varies according to final drive sprocket

The astounding twin-cam ]AP engine, born long after the demise of the old factory, was built for a sport where acceleration counts for everything.