Real Classic - - What Lies Within -

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

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