1967 Bridgestone 350 GTR

One of the most sophisticated Japanese motorcycles of the Sixties

| March/April 2007

Bridgestone 350 GTR
Years produced:
Total production: 9,000 (est.)
Claimed power: 37hp @ 7,500rpm
Top speed: 95mph
Engine type: 345cc two-stroke, air-cooled parallel twin
Weight (dry): 160.6kg (354lb)
Price then: $695 (1970)
Price now: $1,800-$4,000
MPG: 45 (est.)

Accelerating out of a curve with the two-stroke engine revving hard, sun gleaming off the chromed tank and a high-pitched exhaust note providing a vivid soundtrack, it’s easy to understand why the 1967 Bridgestone 350 GTR was widely regarded as one of the best Sixties middleweights around. It’s also a bit sad to think that this model was the high point for a firm that abandoned motorcycle production shortly after it was built.

The Bridgestone 350 GTR was one of the most sophisticated Japanese motorcycles of the Sixties, featuring a disc-valve induction parallel twin engine as well as generally high quality construction. Almost three decades after it was built, this immaculate GTR impresses with its neat looks, crisp performance and reliable handling. Yet only a few years after this bike rolled out of the factory in 1967, Bridgestone not only ceased production of the GTR but gave up making motorcycles altogether to concentrate on the Bridgestone tires for which the Japanese company is still well known.

After riding the twin, that decision seems strange, although it makes more sense when you realize that the little two-stroke was expensive, costing as much as a Triumph Bonneville in some markets. The GTR was good all right, but in most people’s minds it wasn’t that good. Most motorcyclists were unconvinced about the appeal of the relatively little-known Japanese company and its flagship two-stroke twin, with the result that relatively small numbers of GTRs were sold before production ended in 1971.

Induction production
The most notable aspect of the 350 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 being used by rival two-stroke roadsters. Ironically, Bridgestone’s Japanese rival Suzuki had considerable experience racing disc-valve two-strokes, but the firm’s 250cc Super Six roadster, also a two-stroke twin, was piston-ported. Suzuki’s experience dated back to 1961, when MZ factory racer and engineer Ernst Degner defected from East Germany, bringing his team’s secrets with him and passing them on.

Bridgestone’s twin used a disc valve (one for each cylinder) on each end of its crankshaft, with a 26mm Mikuni carburetor bolted outside each valve. Another neat feature was the “piggy-back” alternator, situated above the engine rather than at the end of the crankshaft, making the GTR unit quite slim despite its side-mounted carbs. Peak output was normally claimed to be 37hp at 7,500rpm, although a figure of 40hp was also quoted in some materials. (Most manufacturers were optimistic with power and speed claims, and Bridgestone played that game enthusiastically.)

12/31/2017 9:56:35 AM

The Bridgeton 350 and Kawasaki 350 Avenger we’re about the fastest street bikes from the late 60s both had rotary valve induction which allowed the intake port to open and close at different times. This allowed the bike to be ported wilder while still giving good lower r. p.m. and mid range power. Cycle World managed to get a 13.8 second quarter mile at 94 miles per hour out of the Avenger but they tore it down and found it had been lightly modified with a cut rotary valve. We had a 350 Avenger back in 1968 which we ported to factory A7R Road racer specs. Kawasaki would send a bulletin giving away the secret racing specs to dealers. We built our own design custom expansion chambers for the bike using Cycle magazine formula from Gordon Jennings. We guessed at the rpm we built the pipes for but They were short so probably 9000 rpms. and that Bike was super fast. We raced 10 Corvette quarter-mile and beat all 10 as they went up in wheelspin smoke trying to catch our little oil burner. This bike still exists. .

12/31/2017 9:37:37 AM

The 350 Bridgestone and the 350 Kawasaki Avenger were about the fastest street bikes in the late 60s. they were as fast as many big bikes like Triumph, BSA, and Norton. Both had rotary valve induction which allowed the intake port to open and close at different times. This allowed the bike to be ported wilder while still giving good lower rpm and midrange power Cycle World managed to get a 13.8 second quarter mile at 94 mph with a 350 Kawasaki avenger. We had a 1968 Avenger back in 1968 that we ported to factory A7R roadracer specs. KawasAki would send the porting specs to dealers. We build custom expansion chambers for the bike and we used Wiseco racing pistons and aluminum rotary valves. That Bike was really fast and over the course of the next 10 years it outran 10 Corvettes in quarter-mile drags. Then in 1969 Kawasaki brought out the 500 triple which was piston port but much faster than the 350. .

12/31/2017 9:31:49 AM

The 350 Bridgestone and the KawasAki 350 Avenger were the fast street bikes of the late 60’s. Both had rotary vale induction which allowed to intake port to open and close at different times. This allowed the bike to be ported wilder while still giving good midrange and low rpm power. Cycle World managed to get a 13.8 second quarter mile at 94 mph on the KawasAki Avenger. We had a 1968 Avenger which we ported to A7R specs ( the factory sent the secret porting diagram to dealers). We build our own custom expansion chambers. That bike was fast, beating 10 Corvettes in short 1/4 mike sprints, then in 1969, KawasAki blew them all away with the 500 triple, which was forced to use piston port induction. Because 3 cylinders are better than 2.

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