Development
The Panther sloper engine designed by Granville Bradshaw remained unchanged for 25 years and the 650 cc 'heavyweight' had many of the Model 100 components. The increased capacity was achieved by boring out the Model 100 cylinder by 1 mm and increasing the stroke by 6 mm. As well as increased low speed torque this improved acceleration and top end cruising speed.
This put additional strain on the main bearings and crankcases, leading to reliability problems. The 120 also suffered with lubrication problems from the large sump cast into the front of the engine. Oil was pumped to the top of the sloper engine but return was erratic and oil consumption very heavy. The main shaft was redesigned with larger ball journal main bearings, A smaller exhaust valve combined with a larger Amal monobloc carburettor gave the Model 120 28 bhp at 4500 rpm, an increase of 4 bhp over the Model 100.
Motorcycle sidecars were popular at the end of the 1950s, so the Model 120 was provided with a purpose built sidecar chassis, connected by a three point mounting which provided minimal alignment problems. The sidecar wheel was interchangeable with the motorcycle wheels and had swinging arm suspension with an Armstrong shock absorber. A trailer with a towbar was also provided as an option.
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