The Guy Big J8 was a 8-wheeled chassis that could be purchased as a 24 ton GVW. or a 32 ton GTW. vehicle. Like the other chassis in the Big J range, it was available in a number of forms to suit the requirements of different operators.

The overall length of the chassis was 29 ft 10½ inches, with an overall width of 7 ft 10 inches. The wheelbase was 16 ft 10 inches long. Versions were also available with wheelbases of 14 ft 9 inches and 15 ft 9½ inches.

The Big J8 was fitted with a V6 diesel engine that produced 170 bhp at 2,600 rpm with maximum torque of 370 lbs ft at 1,700 rpm, or the AV 505 (right hand only) which produced 170 bhp at 2,400 rpm, or the AV,471 (right hand only) which produced 150 bhp at 2,400 rpm. A single plate clutch with cushioned drive was fitted.

The gearbox was a 5 speed constant mesh gearbox although a 6-speed constant mesh gearbox with under drive splitter was offered on the double drive V6 model.

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