1967 VELOCETTE THRUXTON

IN DETAIL:

The 1960s and 70s were exciting times in road-racing.

Grand Prix stars competed in nonchampionship international class races on exotic works machines, National Championships were hotly contested in all classes and club racing at scores of tracks was often over-subscribed.

The UK scene was seeing an increase in production racing. The advantage of this class was two-fold: it gave manufacturers a chance to show off their high-performance models at selected races and offered a way to enter the sport without having to purchase expensive ‘real’ race bikes.

Velocette Motorcycles embraced the new class to promote models of their single cylinder 350cc Viper and 500cc Venom machines.these bikes appeared dated, incorporating features of their old single-cam KTT such as: narrow crankcase, dry-sump lubrication, hairpin valve springs, and single down-tube cast lug frame with separate engine and gearbox.

A high camshaft push-rod design replaced the KTT bevel driven OHC.THE narrow pressed-up crankshaft spun on taper roller bearings assembled in the crankcases with a 0.004-inch ‘pinch’ or pre-load. At operating temperature, expansion gave zero pre-load.

An H-section con-rod with a roller bearing big-end and plain bearing little-end supported the cast aluminium three-ring piston. An aluminium cylinder with cast iron sleeve, plus the aluminium head were secured to the crankcase with four through studs. A separate cover incorporating the rockers and enclosing the hairpin valve springs bolted to the head.

The high mounted camshaft was driven by a gear train from the right end of the crankshaft; these gears also drove the rear-mounted magneto. Like the KTT, primary drive was by chain to the clutch inboard of the rear sprocket.

Looking like a museum piece, the cast lug/ tube frame did incorporate a novel solution to adjusting the characteristics of the rear suspension.the tops of the rear shocks could be pivoted around their lower swingarm fastenings from near upright position to a ‘lay-down’ position. Front suspension was by telescopic forks with rebound oil damping, the front axle located in front of the bottom sliders.

Although appearing old-fashioned, Velocette singles had excellent handling and performance. In 1961, a near stock Venom set a world record, averaging over 100mph for 24 hours! High performance parts were available

Mick Ofield