It’s what’s on the inside that counts!
Pete Kelly brings a selection of sectioned and exploded technical drawings of motorcycle engines and other engineering features from the early 1960s from Mortons’ incomparable motorcycling archive.
As the postwar heyday of the 1950s passed into the exciting new world of the 1960s, the art of the skilled technical illustrator was still going strong in the two oldest-established national motorcycling magazines Motor Cycling (1902) and The Motor Cycle (1903), and all but one of the drawings accompanying this article were published between 1960 and 1962.
If editors wanted to show their readers the inner workings of a new four-stroke single or twin, such as the 343cc BSA B40 or unit-construction BSA 500cc A50 and 650cc A65 twins, the illustrators were on hand to do their bidding, and the same applied if the required drawings were somewhat simpler, perhaps illustrating a new frame or clutch mechanism, or the new leading-link front fork and fivespeed Albion gear cluster of Royal Enfield’s 250cc Super 5.
Whether it was a complex racing machine such as the Leo Tonti-designed 350cc Bianchi twin, or Montesa’s simple yet amazingly effective 125cc single-cylinder twostroke; a new 250cc British scooter like the DMW Deemster or Velocette Viceroy; or a sectioned drawing of an entire machine such as the 692cc Royal Enfield Constellation, the skills remained in place to capture every detail.
In The Motor Cycle of March
15, 1962, under the heading This Twin Rivals the Fours, the engine of Tonti’s works Bianchi brainchild was put under the microscope by technical editor Vic Willoughby, whom I was privileged to know as an unforgettable journalistic colleague during the mid-1970s.
After being a successful international motorcycle racer, Vic, who was born in 1914 and passed away in 2000, became The Motor Cycle’s leading technical light as well as being a superb road-tester and all-round reporter from the 1950s until his retirement at the end of the ‘70s, and no Isle of Man TT reporting team was complete without him.
Back in 1962, the two exploded drawings with which we begin this article took up virtually the whole opening spread of Vic’s five-page analysis, which began: “Supposing you were set the exciting task of designing a three-fifty for classic racing. Would you give priority to ultra-light weight, the lowest possible centre of gravity and the smallest bulk – and so sacrifice power by restricting your choice of engine layout? Or would you go all out for power at the expense of the other virtues?
“For make no mistake, you can’t have it both ways. And, with circuits as varied as they are, you would be sticking your neck out to insist that the odds were wholly in favour of either approach.
“Ultra-light weight aids acceleration, braking and climb; a rock-bottom centre of gravity makes for easier cornering; and for a given power, smaller bulk means more speed. Followed to its logical conclusion, the first approach leads inevitably to the low, slender, flat single, weighing just under 2 cwt. Such was the Moto Guzzi layout that monopolised the world’s 350cc championship from 1953 to 1957, the last year the factory supported racing.
“But, for a given calibre of design, engine power goes up with the number of cylinders. Hence the second approach produces, for example, the transverse four exploited so successfully first by Gilera, then by MV Agusta. The reasons you can’t have it both ways are that a four is necessarily bulkier and heavier than a single; and because of its width the engine cannot be slung nearly so low without grounding on corners.
“The choice isn’t easy. So maybe you would try a compromise not yet attempted? That is, design a parallel twin slim enough for low, horizontal installation and as little as possible heavier than a single. Thus you might hope to wring the ultimate potential out of the first approach. Certainly the attempt would be bold. Of course, several factories have plumped for the compromise of a twin; but in all cases the designers considered an upright installation indispensable because of the width across the cylinder heads. Consequently, in weight, bulk, centre of gravity, height and power, the three-fifty twins usually come somewhere between the flat single and the transverse four.
“The latest twin, however, rivals the fours for sheer power. Last year, in its first full season’s racing, it proved to be the fastest three-fifty at many a grand prix, including the TT and the Dutch, Italian and East and West German events. It is the Bianchi – Leo Tonti’s brainchild. But for some minor teething troubles and imperfect handling on bumpy bends, it would have harvested a much richer crop of successes.”
The Bianchi was full of ingenuity and practicality for the GP racing of the day. The valve operation could be changed readily to desmodromic if desired, and ringing the transmission changes between five and six speeds took just a few minutes, with no need to part the crankcase halves. Although the primary drive was by enclosed gears, the reduction could be varied as readily as that of the secondary drive (i.e. chain and sprockets).
Our first drawing shows, among other technical details, an upturned cylinder head with valve seats cut directly into the casting (no inserts used) and the hairpin valve springs, each with 3½ coils on each side, which worked in light-alloy collars.
The valve stems were Stellite-tipped for wear resistance and their guides were full length rather than cut off flush with the port wall. Seen to the left of the crankcase half in the first drawing were the rev-meter drive gear box, foot-change selector mechanism and final-drive sprocket.
In the second drawing, four flywheel discs with integral shafts, two crankpins and a reinforced coupling sleeve incorporating the 24-tooth driving pinion were pressed together to form the crankshaft, the loose disc between the inner shafts forming an abutment for the dismantling tool.
The middle bearings for the crankshaft were housed in a robust casting secured to the crankcase by nine flange bolts, four crankcase-mouth studs and six radial and chordal screws.
Such details were the mere tip of the iceberg in Vic’s utterly comprehensive description of the 48bhp machine on which, in the previous year, Bob McIntyre had finished second at Assen and third at the Sachsenring, and Alan Shepherd had taken fourth place at Monza.
Moving on to classic British single-cylinder motorcycles, a beautiful sectioned drawing of the 343cc BSA B40 Star engine, a unit derived from that of the popular 250cc C15, illustrated a full description of the newcomer in the September 8, 1960 issue of The Motor Cycle – and the enthusiastic road test that followed on January 19, 1961 began: “Study almost any three-fifty of a few years ago and what do you see? In nearly every case you find a model sharing a common frame, wheels, cycle parts and other components with a sister five-hundred.
“In order to withstand the stresses imposed by the bigger engine these components have to be robust – far more so than demanded by the three-fifty. And since favourable power-to-weight ratio is one of the most important factors in the quest for good all-round performance, the three-fifty invariably began life under an illogical handicap. It was robust, yes, but acceleration, economy, braking and handling (and even manhandling) were inevitably less good than they might have been.
“Today, however, the picture has changed, and the one-time Cinderella moves in the royal circle. Three-fifty engines occupy modified versions of 250cc frames, and such a model is the BSA 323cc B40 Star – one of the latest arrivals on the new ‘potent-lightweight’ scene.”
Unlike that of its smaller C15 sister, the new engine was distinguished by the absence of an external pushrod tube, for the rods were enclosed instead in an integrally cast tunnel in the cylinder barrel, and the B40 had a heavier front fork and 18-inch diameter wheels rather than the 17-inch diameter items fitted to the two-fifty.
Top speed in fourth gear was 75mph, and it took the 21bhp newcomer a mean time of 19.6 seconds to cover a standing-start quarter-mile, with a terminal speed of just 66mph.
Notably absent from the report, though, was any lament for the beloved 30bhp 350cc ‘Goldie’ , which like its 500cc counterpart was now living on borrowed time and would be gone forever within the following three years.
It was left to a later newspaper-style issue of The Motor Cycle to correct this oversight some years later by publishing a sectioned drawing of a Gold Star (which we also reproduce for comparison) along with a summary of the ‘Goldie’s’ distinguished history.
Part of the elongated caption went: “The square-fin look appeared in 1954 and, almost completely redesigned, the new motors gave 30 and 37bhp. Both sizes had a new head and barrel, shorter con-rods and mechanical breathers. One notable feature was the switch to eccentric rockers for valve-clearance adjustment.
“The new short-rod 85 x 88mm five-hundred needed oval flywheels to gain piston-skirt clearance at bottom-dead-centre, but later a shorter piston skirt proved just as good – and cheaper. Needle-roller bearings were adopted in the gearbox for 1954.”
By the end of 1957, though, some production of Bert Hopwood’s handiwork was being axed under pressure from BSA’s sister company, Triumph, and for some time afterwards the Gold Star languished almost invisible in the catalogue. Much to the grief of enthusiasts and BSA staff alike, the models finally became the subject of an edict from on high, and emphasis was transferred to the Triumph Tiger
100 twins.
Among the best-looking and most popular single-cylinder lightweights of the time were Royal Enfield’s
248cc Crusader and Crusader Sports, but in the September 21, 1961 issue of The Motor Cycle an unexpected newcomer was announced – the five-speed Crusader Super-5 that featured a new leading link front fork, five-speed Albion gearbox and, like the rest of the Crusaders, a new and wider air-smoothed glass-fibre rear mudguard.
Once again the tech illustrators were called upon to help – not by drawing a complete engine (that had obviously been done before) but instead drawings showing the Super-5’s front fork and that new cluster of gears.
Remaining with Royal Enfield, after the unfortunate demise of Vincent in 1955, the 692cc Constellation became Britain’s biggest twin, and to accompany a road test report in the May 18, 1961 issue of Motor Cycling, a full centre spread was given to a fully sectioned drawing of it.
Seen in full racing crouch on the lusty parallel twin on the embankment at MIRA, and easily identifiable by the triangle on his helmet, was staffman Bruce Main-Smith, with whom I worked as a 20 and 21-year-old junior reporter on the title in 1965 and 1966.
One abiding memory of Bruce was returning with him on the train to London after one of our regular Monday proof-reading sessions at QB printers at Colchester, when the question of expenses arose. “What are those?” I asked naively, having started off on a small local newspaper where, if you even had the audacity to take a taxi to the local magistrates’ court rather than a bus, the office manager would knock it down to the bus fare straight away. “My dear boy,” he replied, “don’t you know you could be sitting on a gold mine?”
In the road test report he wrote: “The 692cc Royal Enfield is the largest-capacity motorcycle currently made in Britain, and with a sports specification including twin carburettors and high-compression pistons, is also one of the fastest machines on the market. Maximum speed obtained at the MIRA test track was a scorching 112mph, the machine rock-steady and the motor running at exactly its rated peak of 6250rpm – claimed output at which is 51bhp.”
As the drawing shows, the ‘Connie’ followed normal Royal Enfield parallel-twin practice in having a crank assembly machined from a single forging, a dry-sump lubrication reservoir incorporated in the crankcase and dual front brakes. Styling-wise, the model featured Enfield’s twin-pilot-light headlamp ‘casquette’ and the new moulded glass-fibre rear mudguard.
In the January 4, 1962 issue of The Motor Cycle, Midland editor Bob Currie heralded BSA’s unit-construction 499cc A50 Star and 654cc A65 Star unit-construction parallel twins by starting his report: “They’re ringing in the New Year at the BSA factory with a pair of brand-new big twins. Low-built and light (some 40lb lighter than the current counterparts), the latest in the Small Heath line-up are packed with power and as compact in appearance as only unit-construction of engine and gearbox can achieve…”
The four-page feature incorporated a sectioned drawing by Lawrie Watts of one of the en ines along with several other tehnical illustrations, one of them a cross-section through the cylinder head illustrating the valve angles and spark plug positioning. Both engines had the same 74mm stroke, with bores for the 499cc and 654cc versions measuring 65.5mm and 75mm respectively. The rocker box was cast integrally with the cylinder head, and the gearbox provided an ‘up-for-up’ movement for the gear pedal.
Two outstanding 250cc scooters of the time provided yet more fodder for the technical drawing teams, the first being the lavish 248cc Velocette Viceroy, described in The Motor Cycle of November 3, 1960 as “a luxurious scooter with flat-twin two-stroke power unit and shaft drive”, but as I alluded to it in my flat-twins feature of only two issues ago, we’ll just concentrate on the drawings this time.
Just a little simpler in concept was the 249cc DMW Deemster that was covered in a three-page feature, including three fine technical drawings, in The Motor Cycle of January 19, 1961.
“A complete departure from the conventional, a machine crammed from nose to tail with novel yet thoroughly practical features,” began the report. “Such is the Deemster, the latest model to come from the drawing board of DMW designer Mike Riley. Powered by the 249cc twin engine-gear unit, it appears to be an amalgam of scooter and motorcycle, yet it is no hastily conceived hotch-potch…”
In fact the machine had been under development for almost two years, its existence kept secret until Mike was completely satisfied that all the bu had been shaken out.
Of particular interest was the all-welded and extremely robust frame featuring 2in-diameter tank and seat tubes and a 1½ in-square-section front downtube. Full legshields, windscreen and carrier were also part of the specification, and there was a choice of electric or kick-starting.
Sadly, despite the promise both held, neither the Viceroy nor the Deemster achieved the sales that might have been anticipated.
Finally, let’s turn to another racing model – this time Montesa’s carefully developed 125cc twostroke Sportsman production racer which, in 1961, developed an impressive 18bhp at 8000rpm. Back in 1956, riding a forerunner of this small and light machine, Marcello Cama had swept quietly to second place in the Lightweight 125cc TT.
Optimising the power characteristics of two-strokes can be incredibly difficult to work out, and ‘the Blue ‘Un’ of April 6, 1961 could have chosen no one better than Hermann Meier to do the maths and write the story.
The exploded drawing on the opening spread shows features such as the crankcase-mouth pillars, piston-skirt cutaways to match the transfer ports and shortening adjacent to the inlet port, steep induction downdraught, I-section connecting rod and open crankshaft balance holes, with a completely orthodox gearbox and clutch.