When one modern bike rider decided to decelerate into a more laid-back lifestyle, a 1930s Sunbeam single provided the perfect vehicle for his transition. Henry Gregson reveals what turned out to be a challenging project…
Anyone who has performed a total restoration, starting with only a box of bits of unknown origin and quantity, knows it’s not easy. Geoff is quite a talented engineer so he was up to the task, and ‘slowing down’ didn’t cover his workshop sessions. He was more than up to the challenge.
The story begins with the purchase at online auction of said box of bits, described as a 1939 350cc Sunbeam B24 high camshaft model, located in Oswestry on the Welsh border. After collecting his new project and arranging the parts on the garage floor to perform a ‘stock check’, it was obvious to Geoff that quite a few parts were missing and that others were badly in need of repair. Both hubs were there, but minus the rims and spokes. The headlight, handlebars, levers, battery cage, toolbox and chronometric speedometer were missing. More importantly, the left-side crankcase was missing a mounting lug, which was going to require fabricating and welding. In the rightside case, the threads for the pushrod tubes were stripped, and so on. Some serious repair work was required on these items; ditto for the inner primary chaincase which required areas of welding.
There was however better news regarding the cylinder, which had been re-bored and came supplied with a new piston and rings. New valve guides were required and, while this work was being undertaken, Geoff took the opportunity to convert the valve operation from hairspring to a more conventional coil spring arrangement.
At the bottom of the engine, the big end was checked and new main bearings fitted. The bad news continued when Geoff found that the oil pump was stuck. Removal required the crankcase to be drilled and then heated before the pump could be punched out. When Geoff took possession of the project, the engine had already been stripped: wonder why? The reason rapidly became obvious when it was discovered that a cam follower foot had sheared off. A new one had to be fabricated, and to complete the job new camshaft bushes were fitted too.
The Burman gearbox was stripped and all bearings replaced as were the clutch plates and all chains. Corrosion had taken a heavy toll of the timing cover and no amount of work could restore it. So a (very) good friend of Geoff’s wrote a software programme and CNC machined a new one out of billet alloy.
And there were moment of good fortune to offset the bad luck – a suitable horn was acquired from an autojumble for the princely sum of £1!
Inevitably, the wiring harness needed to be renewed and one was built from scratch. A reconditioned magneto, dynamo and rectifier were also added to ensure that everything was going to be as sound as possible in the electrical department.
The bike’s tinware had suffered badly and only half of the original rear mudguard remained. This was used as a template to enable a new one to be professionally manufactured, a lengthy process which took in the region of twelve months. A replacement front guard was fabricated at home by taking a standard mudguard and adding side plates and a splash guard to it. Likewise, the exhaust pipe was fabricated from pieces of stainless tubing which were
cut and welded to suit. The silencer proved to be a much easier job as Armours were able to manufacture one.
The battery cage and toolbox were further items that had to be fabricated, these being done at home. Replacement rims were supplied by Central Wheels and, after powder-coating and pin-striping, these were built onto the original hubs. The original tank was also used, but was sealed prior to being cosmetically refinished.
The petrol tank was professionally finished in two-pack prior to the application of the gold lining, decals and final lacquering. All Launching a new and conventional single cylinder range in the age of the first Triumph twins was a bold move. Even more bold when you remember that war was imminent the black paintwork on the cycle parts was re-finished using Simoniz Tough Black gloss aerosol cans which gave an outstanding job – so much so that the Sunbeam won a Best In Show award the first time it was exhibited.
There wasn’t a fairy tale ending to the story, however. As soon as the Sunbeam was back in use, further problems cropped up. It all began OK; on completion the engine started after a dozen kicks. A check for oil leaks failed to highlight anything of concern and a short ride was encouraging, even though the gearbox did seem a bit reluctant to engage first gear.
The Sunbeam’s first proper ride was to the Garstang autojumble. All went well on the way there, but on the return journey the bike seemed to be losing power and much use of the gearbox was needed to coax the bike back home. Investigation revealed that the problem had been caused by the magneto being arrowed to run anti-clockwise and the engine running clockwise! Once that was sorted the engine worked perfectly.
That then brings us to the gearbox selection difficulties with first gear. A stripdown of the clutch indicated that all was fine in that department, so the gearbox had to come to pieces again. This revealed a bent mainshaft which was eventually straightened by the use of a hydraulic press. A further complication arrived after a couple of years’ use, when the protective lining inside the petrol tank separated from the tank so had to be removed and replaced with a different brand.
The Sunbeam company had a reputation for building excellent motorcycles and supported the successful racing of their products. I am sure that many will argue that they were among the finest motorcycles of their time. Originally manufactured by John Marston in Wolverhampton, the Sunbeam brand was purchased by ICI who then later sold it to the Colliers of London in 1937 where it became part of the AMC group of companies until being purchased by the giant BSA concern in 1943.
When this model was introduced in 1938, the range of Sunbeam motorcycles comprised a range of ohv high camshaft models extending from 250cc through to 600cc, with enclosed valve gear and chunky, short pushrods. The three bigger engines shared a stroke of 93mm giving them progressively ‘square’ internal dimensions as capacity increased – quite different to the old marque’s 105.5mm long-stroke sidevalves. The B24’s camshaft was chain driven and tensioned by a Weller chain tensioner; reputed to remove all backlash. The steel flywheels had smooth outer faces, as it was believed that this would reduce oil drag, and the big end was a three row, roller bearing design.
Much work was done by AMC/Sunbeam in Plumstead to maximise smoothness of running and reliability from the sporting engine. Engine rigidity was also helped by internal ribbing. The oil pump was of a geared type as it was believed that provided a larger capacity and a faster oil distribution than a plunger one. The cylinder head was cast iron, two valve, single port and (originally) contained hairpin valve springs. The head and rockerbox were a one-piece casting in order to give better heat distribution and quieter running.
The cost of a new, standard B24 in 1939 was 64 guineas, with the sports model costing an extra 2 guineas. Optional extras including speedometers, luggage carrier, rear chaincase, stop light and pillion fitments.
The Sunbeam was certainly quite advanced when it was introduced, but what is it like to ride now? Geoff says that in his experience the B24 is comfortable for short distances only. The rigid frame and limited saddle springing do not find favour with his back! AMC were all too aware of this shortcoming and introduced a springer frame in the summer of 1939. This used plunger-style suspension units to provide a better ride,
ideal for the sporting gentleman, but very few springer Sunbeams were actually built before hostilities broke out.
Over poor surfaces on modern roads Geoff says that the forks are not really up to the task and have to work very hard indeed. He’s considering adding a second damper which he hopes will assist in providing them with an easier time. The B24S would have been equipped with twin friction dampers as befits its sporting spec – as well as a high level exhaust, slimmer mudguards, and polished cylinder ports and head.
Geoff reckons that the rear brake performance is quite acceptable, however the front one is described as poor. How times change – when it was new the eightinch front brake was ‘a really good stopper’ according to marque expert Robert Cordon Champ.
The B24’s engine performs well and the riding position is good. The four-speed gearbox now works nicely, although he does comment that first and second ratios are very close with a quite large jump then to third. Top gear is ideal for the bike’s 40-50mph ‘very smooth’ cruising speed.
Geoff recommends that anyone who is interested in the pre-war Sunbeams should join the John Marston register. Through their bi-monthly magazine, ‘Beaming’, he has been able to share the costs with other members to have batches of unobtainable parts manufactured. This is obviously a great asset to anyone undertaking a restoration.
Geoff’s initial decision to change lanes in his motorcycling lifestyle seems to have worked out. In fact, he’s now working on two more rebuilds! So much for taking it easy…
The brochure made much of AMC’s all-new engine: ‘The heavily finned cylinder barrel is deeply sunk into the massive crankcase, while the cylinder head is held down by long bolts passing right down into the crankcase casting. This design, coupled with an unusually stiff crankcase assembly with widely space bearings provides great rigidity under load, with correspondingly high mechanical efficiency’
Two line drawings from the archive show the traditional Sunbeam Model 9 motor from 1937, compared with the funky new AMC highcam design
Left: When the bikes were new, the cylinder barrel and head were painted black, as is usual with cast iron. However … to show real class, the edges of the fins were chromed
Right: AMC, home to AJS and Matchless, were keen to preserve the Sunbeam marque identity. They tried hard, too
Sunbeam stripped bare. Most folk enjoy a challenge!
Although the brakes gained much complimentary comment at the time,. The front is fairly feeble. Here they’re enjoying a little refurbishment
One of the few signs of the AJS / Matchless influence are these three acorn nuts, familiar to any owner of an AMC single
Machines of this age are delightfully simple to work on. Mostly…
Three views of the rear mudguard. This is a professional re-making of the original
Handsome, but not wildly effective
Below: The B series of Sunbeam singles was scheduled to continue production in 1940, but the war effort meant that AMC switched to producing G3s instead, and the Sunbeam tooling and production lines were dismantled
Above: Neat electrical installation. All electricals are provided by a Lucas Magdyo, regulated for the lighting by the neat unit mounted behind the battery, while the stout accumulator itself was originally rubber mounted