BSA’s Bonneville Beater

Story by Graham Clayton// Photos by Graham Clayton
June 1 2013
BSA A65 Spitfire

By 1965, Triumph’s T120 Bonneville (reputedly good for 120 mph) was generally accepted as the top sports bike for fast street riders. Other manufacturers had responded with their own super sports road burners, such as the Norton 650 SS and Manxman, and the BSA A65 Lightning, but the Bonneville was still viewed as the must-have bike by most of the “go fast” crowd.

Enter the 1966 Spitfire Mk II, BSA’s “Bonneville beater,” an up-rated A65 model designed with production racing in mind, and meant to challenge the Triumph T120’s top streetbike claim.
BSA had originally introduced the A65 series of OHV vertical-twins in 1962. Unlike earlier BSA 650s, the A65 had an over-square bore and stroke with a displacement of 654 cc. It was also of unit-construction design, with oval-shaped, polished crankcase covers that gave what was called the “power egg” look, popular with many engine designers at that time, but most notably the Italians.

BSA A65 Spitfire The Spitfire engine came in a high state of tune that yielded 54 hp, thanks to the use of 10.5:1 high-compression pistons, hotter cams and a pair of Amal 1-5/32” GP2 carbs that shared a matchbox-type racing float chamber and were fitted with unscreened, aluminum velocity stacks. This was several more horsepower than what a typical T120 produced, and delivered strong acceleration.

A triplex primary chain connected the crankshaft to a wet clutch, and a close-ratio, four-speed gearbox used tall third and fourth gears, spaced fairly close together to facilitate fast, spirited riding while yielding a top speed of around 115 mph (185 km/h).

The A65 engine had a forged, one-piece crankshaft with a central flywheel. The cam was located in the lower crankcase, above and behind the crankshaft at the base of the barrel, and it operated pushrods that were located in a tunnel through the centre of the crankcase. The cylinder barrel was a one-piece, iron unit mated to a single, twin-port aluminum head. This gave the engine a clean look, free of the many cables, oil lines and linkages associated with many motorcycle engines of the day. It looked new and modern.

The Spitfire engine was bolted into a black, new-for-1966, steel-tube frame with a large-diameter, single-backbone tube connected to twin down-tubes front and back to form a full cradle layout. A welded subframe provided support for the dual race-humped saddle, swingarm, rear suspension and fender. The low-slung exhaust used dual tube-shaped mufflers. Both centre and side stands came standard.

The Spitfire suspension consisted of a pair of black telescopic forks and gators and a swingarm fitted with twin, fully enclosed Girling shocks, new for 1966. The suspension was compliant enough for around-town riding, while suitably firm for fast roadwork.

The Spitfires rolled on 19-inch front and 18-inch rear rims laced onto hubs that housed a pair of single-leading, twin-shoe drum brakes. The front brake was a 190 mm unit with five heat-venting holes machined into its left-side cover, while the rear was a conventional BSA seven-inch unit. Chromed, sport-type fenders were used both front and back. Tires were typically Dunlops.

Three different fuel tanks were made available between the home-market and U.S. export models, including a standard, four-gallon (18 litre) fibreglass sport tank for the home market, plus an optional five-gallon (22.7 litre) competition-type fibreglass fuel tank for production racing, and for road-bike use in 1967. For the U.S. market, a 1.9-gallon (8.5 litre) peanut-type tank was made standard; this tank style became popular after its introduction on the 1957 Harley-Davidson Sportster. All gas tanks were rubber mounted.

Engine oil was stored in a rubber-mounted oil tank, located under the double saddle and concealed by one of the two large side covers The bike’s toolbox was hidden this way also. These covers were meant to give the Spitfire a more enclosed and streamlined look, and to obscure the visible mechanical clutter associated with the earlier A10 models.

Model badging consisted of simple decals on the gas tank and the side covers. It didn’t take a lot of spilled gasoline during fuelling to mess these up, as more than a few Spitfire owners learned, to their regret.

Low-rise sport bars were standard on the home-market Spitfires, while U.S. exports had moderate high-rise handlebars. Smith speedometers and tachometers were also standard, as was the small ammeter recessed into the chromed headlight shell. One oversight on the tachometer was the failure to mark it with a suitable redline, at around 7400 rpm or so.

Starting was via a right-side kick-starter and could typically be achieved with two or three kicks when the engine was cold, but could be troublesome when the engine heated up. The electrical system was upgraded to 12 volts on the A65s, and employed a battery and coil ignition system complete with a Lucas RM19 alternator.

The 1967 Spitfire had a 56-inch (1422 mm) wheelbase, and a dry weight of 383 lb. (174 kg). That made it about nine kilos heavier than the T120, which had a wheelbase about 13 mm shorter. To say that these bikes were closely matched would be an understatement.

The Spitfire was felt by many to be more planted at speed and a better handler than the Bonneville, given comparably high-performance tires.
The Spitfire was stronger and potentially faster in top gear, but it vibrated horribly at high rpm, so that steady high-speed running was hard on both rider and machine. Riders who regularly pushed their Spitfires hard would find themselves dealing with cracked frame lugs, split fenders, broken brackets, cracked lightbulbs and other damage.

Some Spitfire owners found that their bikes suffered main-bearing failure at quite low mileage, which could also result in a blown engine.
The basic A65 engine was designed for strong midrange power and low-speed tractability. The A65 Spitfire was something quite different and required some straightforward changes. Factory insiders who raced Spitfires soon learned that they needed stronger crank bearings, a better oil pump and a modified clutch that could handle high rpm running.

With these modifications in place, people who raced Spitfires won and placed in many events. A factory-prepared Spitfire finished third in the 1968 Isle of Man 750 cc Production TT race behind a pair of 750s, and was clocked at 132 mph during the race. Sadly, BSA never applied these race-proven changes to their production machines.

It was only after the Spitfire model was discontinued at the end of 1968, replaced by BSA’s new supersport model, the 1969 750 Rocket III triple, that a group of BSA enthusiasts started offering aftermarket kits to fix the bearing, oil pump and clutch problems that were the Achilles heel of the Spitfire.
Through 1967 and 1968, BSA started detuning the Spitfire, introducing the Mk III in 1967 and the Mk IV in 1968. The engine’s compression ratio was first reduced from 10.5:1, to 10:1 in 1967, and then to 9:1 in 1968 (the same as the old 650 Lightning).

These changes, along with the replacement of the GP2 Amals with new 32 mm Amal 932 concentric carbs fitted with circular air filters, reduced the model’s horsepower to 53 hp and then 52 hp, but made the bikes much easier to live with.

The carb change seemed to largely do away with the previous model’s reluctance to start when hot. Fuel economy also improved, allowing decent long-distance range on a tank of gas, unless the bike had the smaller, U.S. market peanut tank, in which case fuel stops would be much more frequent.

The biggest changes to the Spitfire came in 1968, when the Mk IV was fitted with full-width hubs and a new eight-inch, double-leading, twin-shoe front brake, exposed-spring Girling shocks with chromed uppers, and new fork legs with split ends that clamped hard onto the front-wheel axle.

The Mk IVs, of which only 471 were built in 1968, is widely regarded as the best and most sought-after version of the Spitfire, though it’s worth noting that more than a few Spits actually built in 1967 were converted to 68-spec before they were sold.


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