Stroked 600 with downdraught head and single carb. Lacks top end power but made up for by useful gobs of torque lower down the range and excellent gearchange action, plus Slimline frame/Roadholder fork handling. Vibes can be vile above 90mph, with fillings falling out and bits cracking up. Those rebuilt with improved modern engine components can be very good indeed, some excellent examples on the private market.
1960-63 646cc(68x89mm) OHV twin 40hp 110mph 60mpg 400lbs