BSA Golden Flash A10



Make Model

BSA Golden Flash


1949 - 61


Four stroke, parallel twin cylinder, OHV


646 cc / 39.4 cub in.
Bore x Stroke 70 x 84 mm
Carburetor Amal
Cooling System Air cooled
Compression Ratio 7.2:1
Lubrication Dry sump
Exhaust Twin, stainless steel


Lucas magdyno



Starting Kick start

Max Power

26 kW / 35 hp @ 4500 rpm
Clutch Multi-plate with bult-in cush drive


4 Speed
Final Drive Chain
Gear Ratio 1st 11.68 / 2nd 7.96 / 3rd 5.48 / 4th 4.53:1
Frame Welded seamless steel tubing with duplex downtubes and full cradle engine support, bolted-on rear sub frame

Front Suspension

Telescopic forks with coil spring - hydraulically damped

Rear Suspension

None or Plunger-type, with shock and rebound springs, undamped.

From 1954: Swinging arm

Front Brakes

Drum, 8 in.

Rear Brakes

Drum, 7 in.
Wheels Steel, wire spokes

Front Tyre

3.25 x 19 in., ribbed

Rear Tyre

4.00 x 19 in.


Length:  2100 mm / 84.0 in.

Wheelbase 1391 mm / 54.75 in.
Dry Weight 170 kg / 375 lbs

Fuel Capacity 

16 L / 4.2 US gal

Top Speed 160 km/h / 100 mph
Standing Quarter Mile (400 m) 16 sec.

In May 1948, Hopwood joined BSA. Briefed specifically to create a competitive parallel-twin, the internally designated A10 model was based on an earlier A7 design by Page and Bert Perkins.[3] After BSA took over Triumph in 1951, Hopwood returned to Norton.

Early Golden Flash A10s had frames with either no rear suspension, or plungers, and had a semi-unit engine and gearbox, with the gearbox bolted to the rear of the engine.

Launched in October 1949, the A10 Golden Flash was a new post-war design, with most of the difference to the A7 being in the engine. Increased to 650 cc (40 cu in), it encompassed revised castings for the cylinder head and rocker box, and a cast-in carburettor manifold. The frame was available in rear rigid format, but the more common option was the then new plunger suspension, specifically designed for overseas export. BSA was a manufacturer who focused on machines for the working man, and so the design incorporated two practical use features: a hinged rear mudguard, designed to ease rear wheel removal; and a semi-unit engine and gearbox arrangement. The semi-unit power train enabled the primary chain to be adjusted via a slipper within the primary chain case.

The large carry over of parts from the A7 had the advantage of greater reliability, as it minimised the risk of any new technical problems. Launched in a new form of gold colour, the combination of reliability and marketing made early exports possible, with 80% of production destined for the USA. The result was long delivery times for British customers, who were offered a more practical and less stand out black.

Although never designed as a fast machine, the Golden Flash was nonetheless fast for its time and competitive with the Triumph Tiger 100, achieving over 100 miles per hour (160 km/h) in tests in 1950, and covering a standing quarter mile (400 m) in under 16 seconds.[6] Its gold colour also made it a marketing and sales success, out selling the red Speed Twin and the later blue-grey Triumph 6T Thunderbird, which was resultantly revised the following year into polychromatic blue.

However, BSA were concerned about a tendency for wear in the rear, plunger suspension, leading to uncertain handling. In 1954 it revised the model with a modern swinging arm, and the hinged mudguard was deleted. The semi-unit power train was also abandoned, making for a more involved procedure for adjustment of the primary chain, it offered benefits in the areas of the clutch and gearbox internals. The revisions also allowed for the launch of more sporting A10 Road Rocket.

In 1956, alloy brake drums were fitted as standard which both reduced unsprung weight and increased stopping power.

In 1957, the 40 bhp (30 kW) 105 miles per hour (169 km/h) BSA Super Rocket was launched for the 1958 season, with an Amal TT carburettor and high lift cam.

The final A10 development, and fastest in terms of power and performance, the 1962 BSA Rocket Gold Star, which fitted a tuned A10 Super Rocket engine into the well proven BSA Gold Star single frame. The result was a good handling fast bike that was a true classic. Current demand for the Rocket Gold Star, or "RGS", is such that non-RGS A10s are sometimes sold as originals, having been modified using suitable alternative parts.

With the launch of the new unit-construction (combined engine/gearbox) BSA A50 and BSA A65, the A7 and A10 were discontinued at the end of 1962 but the Rocket Gold Star continued into 1963 production.

Article from Two Wheels:

The BSA A10 650 twin, to give it the most boring title in motorcycling history, may not be a classic bike, says LESTER MORRIS (intent on beginning yet another argument) but back in the early '50s it was a state-of-the-art machine, solid as a rock, reliable as rain on your holidays and today a mechanical relic that may well be dragged screaming back onto the drawing boards because it used so little fuel. But there's no questioning one thing: It was a motorcycle in the Jbesr British tradition. . .

ASK ANYONE you know to name the best-known British vertical twin, and they will say "Triumph", and of course they would be right. It was popular with enthusiasts and police alike, and it was a perfectly acceptable motorcycle. But it was by no means the only true British vertical twin — BSA built a four-stroke vertical twin which was at once similar in design philosophy to the Triumph and yet very different in its basic specifications.

Initially a 500, the BSA grew to 650 cub cm in late 1950 and retained this engine size through design evolutions which saw Sports and Super-Sports versions and finally, a new unit-construction model  the A65  which appeared in 1965. In the late '50s to very early '60s, the BSA Golden Flash (even though later examples were painted black) was used by the NSW Police Department as a sometime solo mount and a frequent sidecar combination.

It was only the introduction of the Mini-Minor which prompted the local police to abandon the BSA and J.G. Murphy chair — that little rollerskate of a car rang the death-knell of motorcycling in general in the early '60s.

The Golden Flash BSA is not really a "Classic" motorcycle, but it certainly provides a prime example of the state of the art of British twins of that period. Like many of the machines which flourished then, this model is currently almost extinct — do you ever wonder where they all went? — but the few which are to be found are usually in concours condition.

The subject of this particular test, a 1953 model belonging to Col Brenchley, is all that and more. In fact, thanks to modern baking and plating techniques, the overall finish of the old Beeza is probably better than it was when it was brand new! Understandably, the gleaming and chrome-bright bike has won many awards.

From the inside
Typical of its era, the Gold Flash is powered by a long-stroke pushrod-operated overhead-valve engine, with a primary drive by duplex chain to a separate four-speed gearbox. The chain is enclosed in an aluminium case on the machine's left side, with the lower run of chain immersed in oil. The clutch, though contained within the oil bath chaincase, is essentially a dry component. Or damp might be a better word, which could be easily proved by the incautious owner who would bring about instant clutch-slip if he over-filled the case during service.

The BSA twin engine was referred to as a semi-unit construction type because the gearbox was bolted rigidly to the rear of the crankcases, instead of being mounted separately behind the engine, like most of its counterparts. Two bolts locked the gearbox to the rear of the frame as well, the essential primary chain adjustment being carried out by an external grubscrew which applied pressure to a slipper-tensioner within the chaincase.
Naturally, unlike current designs, the gearbox carried its own oil supply, as did the primary chaincase. In the dry sump engine — again unlike modern motorcycles  the oil was carried in a separate container attached to the frame under and at the back of the fuel tank.

A single camshaft at the base of the cast-iron cylinder barrel is driven by gear inside the timing case, while the outer timing case contains the gear drive for a twin magneto and a six-volt generator — the latter driven by a tiny 8x3 mm roller chain about half the size of the tiniest overhead-camshaft drive chain.
The Gold Flash cylinder head is cast-iron, with an integrally cast inlet manifold. A single carburettor is used; the pre-Monobloc type with a fuel-raising "tickler" on the separate float bowl.

Sometimes an induction bias occurred on BSA twins, with a slightly greater charge being fed into one or the other of the cylinders — probably due to an unsquare mounting face which pointed the carby more towards one pot than the other — though there was a tapered anti-bias gasket which you could fit to overcome this odd problem.
Apart from this quirk (which never manifested itself with any of the other twins) this staunch British engine was a solid and reliable mount which could cover prodigious distances without much more than the simplest of routine maintenance.

The bottom half of the engine was sturdy, with a bolted-up crankshaft and split connecting rods with white-metal slipper bearings. A large ball bearing was mounted on the drive side, while the more lightly-stressed timing side was mounted in a large white-metal bush. This bush needed to be replaced on occasion, even though it was fed the first breath of fresh, clean oil from the gear-driven pump mounted right alongside it.
An advantage of the semi-unit construction design, and the basically simple overhead valve layout, lies in the ease with which any reasonable owner can carry out servicing and large repair jobs without removing the engine from the frame — an advantage not always enjoyed by owners of more complex, modern motorcycles.

The power unit is bolted to a duplex-down-tube frame with telescopic front forks and the dated plunger rear suspension which, on this particular model, was almost at the end of its run; the much better swingarm rear The "ugly" side (at top) has the driveline. The large alloy case contains the primary drive chain and clutch, while the counter- I shaft sprocket on the gearbox mainshaft carries the final drive from behind the clutch. Note the six-volt battery and horn, ' both of which live in the open air. On the timing side (above) the six-volt generator mounts in front of the crankcase under the exhaust pipes, with the twin magneto behind the cylinder base. The gearbox is remote from the power unit and is driven by chain from the engine. Oil for the engine is carried in the tank directly above the gearbox. suspension, in such universal use today, was first adopted on the Golden Flash in late 1953 as a precursor of what was left of the model's production run.

A very neat nacelle contains the simple speedo, lightswitch, ammeter combination which was all one ever had — or needed! — on machines of that era. Blinkers were not used, of course, though their great boon to safety is currently unarguable, and most riders could manage to find neutral gear without the aid of a little green light in those days of yore!

Ups and downs
Plunger rear suspension was used on very many British machines from the immediate pre-war era to the early '50s, even though some makes — notably Royal Enfield, AJS and Matchless — ran swingarm rear suspension on some models from 1949. The advantage of the plungers is in keeping the drive chain in a constant tension. Since little more than up-and-down movement is allowed, chain tension remains reasonably constant.
The later swingarm suspension moves the wheel through an arc, playing havoc with chain tensions (and life), though the extra suspension movement and rigid wheel location allows greater comfort and much better handling than plungers ever could.

On smooth road surfaces the Golden Flash handles well and is comfortable enough, but the limited travel of the rear suspension makes for a choppy ride because of its undamped springs and the firm dualseat; arguably, the plunger set-up was at its best when augmented by a single, spring saddle to take the edge off bumps and potholes.

However, the machine does not handle well by modern standards when ridden briskly over rough surfaces — and again this is due almost entirely to the rear suspension. The axle is located within a pair of springs either side of the frame and they can allow the rear wheel to get out of whack with the front end when it moves through its 55 mm or so of travel. This happens because the spring tensions or frictional loadings may cause one side of the plunger system to move further than the other, cocking the wheel to one side and altering the geometry.
The bike will drop readily into corners, though the BSA centrestand does not allow it to be dropped over too far. It will track very securely, although a dip in the road or an unexpected hole will catch the rear wheel on full bump, resulting in a sharp tug at the handlebars.

I must say I had almost forgotten this trick and it is only by making this sort of comparison that one can see how much better the swingarm rear set-up really is  it is not yet perfect, but the pivoting-fork suspension in current use is at least acceptable and allows safe handling at most speeds this side of the ridiculous. By contrast, the BSA front forks are excellent and certainly on a par with the best in current use, but the old BSA trait of heaviness at the steering head is still evident. It's hardly serious, but the bike tends to teeter on the centre of the tyre tread when upright and at speed, which is a mite disconcerting. Somebody once referred to this as the invisible cannonball syndrome, as though a couple of these projectiles were attached by ropes- to the steering head where they flop about willy-nilly.

This syndrome is by no means unknown to some of today's much vaunted motorcycles, in particular the first Z1R Kawasaki models and the pre-1975 BMW short-wheelbase 750s, so it may not qualify as a fault — at least not by direct comparison with machines which should not suffer from this 100-years-old quirk!

Speaking of comparisons between the old and new, for those of you who may find something faintly familiar about the A10/BSA, might I draw your attention to the first of the vertical-twin Kawasaki machines? Here is an engine that is a dead-ringer for the pre-unit BSA, right down to oil feeds to overhead rocker gear and the separate gearbox. A copy, in fact, of the later Road Rocket sports model. At least they had the decency to wait until the new engine appeared in 1965 — but little attempt was made to disguise the origins of the model. Though the engine differed in the bottom-end (notably in the adoption of a ball bearing where the timing side main was fitted) and a more bulbous timing case resulted, the castings were almost identical and the general remarkably similar.

It's funny how you get used to things, and it is only when you have to make do without them that their great effectiveness is realised. It was never more obvious than with the BSA, which has no mirrors! Mirrors were not required 25 years and more ago, and neither were stoplights — or blinkers, as I've mentioned — and the lack of them makes riding in traffic a pretty nervous business. You can't tell who is breathing down your neck, and you can't tell where they are if they are!

Starting the bike took me back to years ago, for the old Amal carburettor, with its separate float bowl and fuel-raising "tickler", needed to be flooded to provide juice for the long kick, and 325 cm3 of cylinder is not as easy to punt over as it once was. Perhaps the Golden Flash BSA is getting a bit old for that sort of thing.

Aren't we all!
The donk needed several kicks to bring it to life, and it idled in the typically lumpy way of that period. Coming onto the needle at about one third throttle and running there made for some snatchiness, but this could be tuned out without too much hassle if it annoyed; again reminiscent of the type of carburettor which was used at that time.

Vibration was, of course, part of the engine design, as it always was and always will be in vertical twins. There is no way it could be described as unpleasant. Rather was it a sign of an honest engine working at its task, the essential changes of engine masses inevitably resulting in transferring of shock loads to frame and rider.

There were some machines which made a feature of heavy vibration, but the BSA twin, at its best, was not one of these.
Modern motorcycle designers go to great pains to mask the inherent vibration by building shafts which revolve in odd directions, but the initial vibrations are still there, masked by other vibrations which are said to cancel them out. Perhaps it would have been nice to have enjoyed this feature on the early British twins but it was apparently deemed unnecessary. Or perhaps they didn't think of it.

In '71 the Flash came alive again. This is Kawasaki's WS1 BSA copy. Enthusiasts of the day said it didn't leak or vibrate as much as the Beeza. They were wrong!

A bike of its time
The Golden Flash BSA is a perfect example of a mid-'50s large-capacity British vertical twin, the type of machine much-maligned by modern riders who have perhaps never seen one of them, and almost certainly never ridden one.

It would be a surprise to many to find out that the drum brakes are brilliant, the alloy-backplate 200 mm front one in particular. The wide shoes and quick-opening cam certainly help, and so too does the bike's comparative light weight. That front anchor is very powerful and progressive in action, with little hand pressure required. The 175 mm rear drum is equally efficient, and both brakes enjoy the priceless boon of being just as effective in the rain as they are in the dry. There are not too many modern motorcycles you can say that about!

The virtues of the BSA twin have been spelled out and so too have its vices, but it remains a prime example to be stacked up against the models from Japan with similar engine capacities, and of course against the survivor . . . the Triumph Bonneville. It shows up well in direct comparison, quite apart from its comparatively leisurely performance (though it was quick enough in its day) it was dead reliable, its handling and comfort were acceptable enough and its fuel consumption was better then — though we hardly knew it at the time — than many a 250 motorcycle today.

As a point of interest, the test model leaked not a single drop of oil on test, and remained in showroom condition throughout. With the accent very much on fuel consumption as we head into the '80s you can't help but wonder whether the large-capacity twins will once again appear with low compression pistons, single carburettor, "soft" camshafts, small valves and modest power outputs.

And if they do — which seems more and more likely — how well would the old BSA then stack up against its modern counterparts?
Quite apart from its rear suspension, it would come through very well, very well indeed.

Source Two Wheels 1981