1978 JAP 500 Speedway Twin-Cam
1978 JAP 500 Speedway Twin-Cam
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 JAP company had closed down, but Alec Jackson, a former rider, revived the manufacturing of JAP 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 (80×99mm) JAP engine, he amended the bore and stroke to 84×90mm.
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 gear driven, 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 even for the rare twin-cam.
SPECIFICATIONS
Engine: 499cc (84×90mm) air-cooled single- cylinder 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 JAP engine, born long after the demise of the old factory was built for a sport where acceleration counts for everything.
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Hello!
I have one of this engines and it is allmoust brand new but the frame it is assemled in is not the best. The number of the engene is 4V 78588.
I can send picturs and I am selling this engine for a good price.
Best Regards,
Fredrik
Fredrik,
I am interested in buying your 4 valve engine and possibly the frame.
Please email details.
rogerbutt47@optusnet.com.au
Regards,
Roger Butt
Sydney, Australia
I have put you two in touch with each other. I hope you can work out a deal!
I have one of these engines in a Rotrax frame it look s very similar to the picture. I inherited it with the engine in a box, I have a ssembled it but I need information on the cam/ignition timing, if you could help with this I would appreciate it. I would be interested in any other information that is available also. Thanks
Steve Milne
Atherton Queensland
Australia