BSA/TRIUMPH 750-3 – THE MECHANICALS
In developing the ohvt 150 in-line threecylinder engine for its debut season in AMA road racing in 1970, Doug Hele and his men in the Meriden factory race department drew on their valuable experience gained working on the company’ s 500 cc twin-cylinder engine, which w had twice won the Daytona 200-miler in n 1966 and 1967, as well as given a good acccount of itself in 500GP races. With the stock 67 x 70mm 741cc triple es ssentially comprising an extra cylinder hung on the side of triumph’ s twin-cylinder ohv motor, m the potential for crossover engineering was w obvious. Firstly, they maxed out the stock 741cc engine to the 765cc capacity permitted under AMA rules via the maximum allowed + 0.040- 0 inch overbore which in theory allowed damaged cylinders to be bored out and re e- used.this entailed homing the austenitic ca ast iron liners to accept specially made high-coompression 11:1 cast pistons for dimensions off 68x70 mm, mounted on stock con rods which were polished and matched. these were fitted to a carefully lightened stock 120° crankshaft, with triplex chain primary drive retained, matched to a five-speed close-ratio Quaife gearbox replacing the stock four-speeder, with a lightened single-plate Borg & Beck diaphragm clutch. TH6 cams first developed in 1968 for the 500GP twin were used with larger-radius lifters, with stock cylinder heads fitted with tougher valve seats carefully ported, and the combustion chamber volumes measured and matched. Three 13⁄ 16in (30mm) smooth-bore Amal GP carburettors with a single separate float chamber feeding all three carbs were initially chosen for greater power at full throttle, such as wide open on the banking. Ignition was supplied via a special Lucas energy transfer system,with an alternator feeding remote contact breakers without the need for a heavy wet-cell battery, and with the three coils mounted in a separate carrier adjacent to the timing chest.the 3-1 exhaust
system employed a long megaphone measuring four inches / 100mm wide at the exit, a system designed by Hele himself who further refined this on the dyno with a gain of 4-5bhp over three separate pipes.the resultant engine delivered 81bhp at the crankshaft, at 8200rpm. Nearby chassis fabricator Rob North was responsible for designing and manufacturing the chassis, based on a design he had already created in conjunction withtriumph tester Percytait.the resultant bronze-welded duplex frame in Accles & Pollockt45 chrome-moly steel tubing carried a modifiedtriumph 15⁄ 16th inch (33.3 mm) telescopic fork with welded-on brake caliper plates. Owing to a mistake with a prototype chassis component that had been supplied to him inadvertently, the first-series 1970 North frames had a tighter 26º head angle, later (for 1971) kicked out to 28º with 121mm / 4.75in of trail. Coupled with the taper-section tubular steel swingarm carrying twin Girling shocks, this resulted in a 1450mm / 57in wheelbase and a 50/50% disposition of the 180kg / 396lb weight, measured with oil, but no fuel. For 1970 the bikes had a 250mm four leading-shoe Fontana front drum brake cast in magnesium, with a single 10in / 254mm cast iron rear disc sourced from AP Lockheed, which transpired to be atriumph Herald car component, suitably machined down! For 1971 though, the Fontana drum was replaced by two more Lockheed disc brakes, after North redesigned the chassis to incorporate the correct front end geometry, and at the same time lowered the steering head 2in / 50mm for a better weight distribution as well as reduced frontal area for increased top speed, and also widened the fork to give space for fitting the disc brakes. The resultant chassis was also lighter than before, scaling 35lb / 16kg in bare metal. Initial attempts to run plasma-sprayed aluminium discs for reduced unsprung weight were aborted after they expanded under heavy braking of the kind needed at Daytona from absolute top speed down to second gear twice per lap at the chicane and turn one, resulting in the brakes locking. After Hailwood was lucky to survive a highspeed crash caused by this, they were replaced by cast iron rotors similar to what had been used at the rear of the 1970 bikes.these the first factory road racers to use disc brakes all around. For 1971, besides the brake changes and the new Lowboy frames with the oil cooler moved to the front of the new Letterbox fairing, other improvements included the drive to the points cam now being taken from a quill drive whose inboard end was pegged inside the hollow exhaust camshaft.this feature absorbed torsional vibration and flexing, giving more consistent ignition timing, especially when used with USmade Bendix points which were more reliable than the Lucas alternative. Power was increased to 84bhp at 8500rpm (again at the crankshaft) with a newth13 inlet camshaft, squish pistons and suitably modified heads. Carburetors were now 30mm Amal Mk.1 Concentrics for a smoother power delivery, while magnesium primary cases and wheel hubs together with titanium fasteners reduced weight.titanium was not legal for axles, but with iron plugs at each end, they passed AMA inspection, which consisted of poking a magnet at one end of an axle! Further weight reduction came from plasmasprayed aluminum-alloy disc brakes, making these the first factory bikes to use disc brakes all around.
Right: Mike Hailwood on a Lowboy chats to jim Rice on a Highboy. It’s 1971 at Daytona. Left: L-R Don Emde, Dick Mann, Dave Aldana and Jim Rice on BSA-3S in 1973.