The GTR was one of the most sophisticated bikes of its day, featuring a disc-valve induction parallel twin engine as well as a generally high quality of construction. Decades after it was built, this immaculate GTR impresses with its neat looks, crisp performance and reliable handling. Yet not long after this bike left the factory in 1967, Bridgestone not only ceased production of the GTR but gave up making motorbikes altogether, to concentrate on the tyres for which the Japanese company is still well known.
The most notable aspect of the GTR’s 345cc parallel twin engine was its rotary disc valve induction system, which allowed much more precise control of gasses than the more simple piston-ported design then used by rival two-stroke roadsters.
Bridgestone’s twin used a disc valve (one for each cylinder) on each end of its crankshaft, with a 26mm Mikuni carburettor bolted outside each valve. Another neat feature was the ‘piggy-back’ alternator, situated above the engine rather than at the end of the crankshaft, which allowed the GTR unit to be quite slim despite its side-mounted carbs. Peak output was normally claimed to be 37bhp at 7500rpm, although the figure of 40bhp was also quoted in places.
Lubrication was by a Yamaha-style pump-operated system, to which the Bridgestone added the refinement of inspection windows for both engine and gearbox oil. Similarly the GTR impressed with its six-speed gearbox, and with its facility to swap the gearlever and rear brake pedals to give a left- or right-foot gearchange, both of which were commonly used at the time.
Its twin-downtube steel frame, gaitered front forks, chromed twin shocks, and 19-inch wheels with drum brakes front and rear were very much standard fare when the GTR was first sold on the American market in 1966. Visually it was similar to its 90cc and 175cc siblings, and there were also models with off-road styling, notably the 350 GTO which was built in even smaller numbers than the GTR.
Engine: Air cooled two stroke parallel twin
Power: 37bhp @ 7,500rpm
Transmission: 6-speed, dry clutch
Carburation: 2x26mm, Mikuni
Front suspension: Telescopic, no adjustment
Rear suspension: Twin shock absorbers
Courtesy: Carole Nash