AJS POR­CU­PINE

g nick­name – and although de­vel­op­ment took place slowly rather than sharpish, it won the first 500c World Cham­pi­onship in 1949 be­fore this dis­tinc­tive racer’s ca­reer was punc­tured four years later

Classic Bike (UK) - - Contents - WORDS: PHILLIP TOOTH, PHO­TOS: GREEN­ING AR­CHIVE, FOTTOFINDERS

De­vel­op­ment story of the spiky-finned, 500cc E90 twin Grand Prix bike

‘WHAT STOOD OUT WERE THE SPIKY FINS ON THE CYLIN­DER HEADS; THEY WERE 5½IN LONG’

The gentle­men of the press had to wait to get their first look at AJS’S new race bike, just a few weeks be­fore the 1947 Isle of Man TT. They had been in­vited to As­so­ci­ated Mo­tor Cy­cles’ fac­tory in Plum­stead to view it, but sales man­ager Jock West had to apol­o­gise for the de­lay – the 498cc E90 par­al­lel twin was still on show in the can­teen, where the work­ers were get­ting their first sight of the fin­ished mo­tor­cy­cle.

Like the work­ers, the jour­nal­ists were im­pressed with what they saw. While the fac­tory Manx Nor­ton was stuck with plunger rear sus­pen­sion, the AJS fea­tured a swingarm rear end and te­le­scopic forks, with a full cra­dle frame con­structed from a mix­ture of round and oval-sec­tion tube. The unit­con­struc­tion en­gine was laid nearly hor­i­zon­tal in the frame to give a low cen­tre of grav­ity, with the cylin­ders canted up at 15°. Bore and stroke were al­most square at 68 x 68.5mm and the pis­tons rose and fell to­gether, fir­ing at 360° in­ter­vals. Although the gen­er­ously-finned cylin­ders were sep­a­rate, there was a one-piece cylin­der head. Sodium-cooled ex­haust valves and stan­dard in­let valves were closed by hair­pin springs. Drive to the two widely-spaced over­head camshafts was by a train of eight gears run­ning on nee­dle roller bear­ings, hid­den by a Y-shaped Elek­tron cover. The Lu­cas rac­ing mag­neto was driven by the first gear in the tim­ing train. Cam boxes were also cast in Elek­tron, with that same mag­ne­sium al­loy used for the com­bined crank­case and gear case, along with the bolted-on sump.

The crank­shaft, a one-piece forg­ing, was sup­ported on a sin­gle plain bear­ing in the mid­dle and roller bear­ings at each end. The con­rods were forged in RR56, a high-strength al­loy de­vel­oped by Rolls-royce for air­craft en­gines, and ran on Van­dervell shell bear­ings. Pri­mary drive was by two straight-cut gears, so the en­gine ran back­wards. The four-speed close-ra­tio gear was a con­ven­tional Brit de­sign, ex­cept that it fea­tured a cross-over drive so the rear chain was on the right side. A mas­sive clutch was mounted out­side the drive case to keep it run­ning cool – it would spin at 80% of en­gine speed. One jour­nal­ist noted that the clutch was so lib­er­ally drilled for ven­ti­la­tion it looked like a crum­pet. While most of the en­gine cas­ings were cast from mag­ne­sium al­loy, the heads and bar­rels were made in alu­minium al­loy. Be­cause the heads and cam boxes shielded the cylin­ders from the air flow, they fea­tured gen­er­ous lon­gi­tu­di­nal fins. But what re­ally stood out were the spiky fins on the cylin­der heads. Those spikes were 5½in long and looked like por­cu­pine quills – so it wasn’t long be­fore the E90 be­came known as the Por­cu­pine.

It was an open se­cret that the E90 Por­cu­pine was orig­i­nally de­signed to be su­per­charged. The laid-down flat-twin con­cept was the idea of Joe Craig who had left Nor­ton in Jan­uary 1939, fed up that it had shelved its rac­ing ac­tiv­i­ties, to go first to BSA and then AMC. Craig was a bril­liant de­vel­op­ment en­gi­neer but no de­signer, so the E90 was laid out by Vic Webb and Phil Irv­ing. Un­for­tu­nately for AMC, Craig re­turned to Nor­ton in 1946 when they made him an offer he couldn’t refuse, and then the FIM banned su­per­charg­ers.

The wide valve an­gles and hemi­spher­i­cal com­bus­tion cham­ber lim­ited the com­pres­sion ra­tio in the orig­i­nal de­sign, which wouldn’t have been a prob­lem if part of the com­pres­sion was pro­vided by the su­per­charger. But to com­pen­sate for the lack of a blower, the E90 had to use forged He­po­lite pis­tons with high domes to in­crease com­pres­sion, which meant that the space be­tween pis­ton and cylin­der head was now shaped like the thin

rind of half an or­ange and the fuel would burn slowly and in­ef­fi­ciently. Long, curved in­let tracts, de­signed for their large vol­ume to buf­fer the su­per­charged in­duc­tion, were also a ma­jor headache and as a con­se­quence the Por­cu­pine strug­gled to car­bu­rate cleanly through­out the rev range.

The 1947 Se­nior TT didn’t run to plan for the AJS team – hardly sur­pris­ing when you con­sider that the two bikes had never been rid­den be­fore they reached the Is­land. Jock West, who was works rider as well as work­ing in sales, strug­gled to bump sta t his be­cause of oil drag in the plain bearin ef­fect, and then his clutch started to sl rid­ing, but had to call into the pits for taken 1hr 24min. Mean­while, Les Gra rid­ing with a se­verely dam­aged left han place, but his chain jumped the sprocke to push in from Gov­er­nor’s Bridge to cla fin­ished 1-2, with Ve­lo­cette in third plac fixed, West had made the sec­ond-fastest 40bhp Por­cu­pine, only 3sec slower than A

Re­li­a­bil­ity prob­lems would slowly be so Por­cu­pine E90 would go down in his­tory won the first 500cc World Cham­pi­onship got off to a bad start. With just two miles a two-minute lead over Harold Daniell’s N noth­ing could stop an AJS vic­tory in the Se mag­neto’s long drive shaft sheared and Les push home to a heroic 10th place. He also f Bel­gian GP. But with wins at the Swiss and U a sec­ond place at the Dutch, Gra­ham did eno the world ti­tle by one point ahead of Nello P Gil­era Four. AJS also won the con­struc­tor’s c

The Por­cu­pine lost its spikes in 1951 when cast­ing for the cylin­der heads was ditched in f two sep­a­rate cast­ings with hor­i­zon­tal, transve

bet­ter com­bus­tion cham­ber shape and power climbed from the orig­i­nal 40 to nearly 50bhp.

In­stead of be­ing mounted in the con­ven­tional po­si­tion un­der the seat, the oil tank be­came a long boat-shaped mag­ne­sium al­loy cast­ing bolted un­der the en­gine. The wheel­base was short­ened by an inch, and the forks low­ered by the same amount, but th­ese were mi­nor mod­i­fi­ca­tions. Only four E90 Por­cu­pines had been built be­fore AJS pro­duced an up­dated ver­sion for the 1952 sea­son.

De­signed by HJ ‘Ike’ Hatch, the E95 would still be a unit­con­struc­tion twin with the same 68mm bore, but stroke re­duced by 0.25mm. The Y-shaped gear train to the over­head camshafts re­mained, but the bar­rels and heads (two sep­a­rate cast­ings) were in­clined at 45° to the hor­i­zon­tal, which al­lowed the area around the ex­haust ports to run at a much cooler tem­per­a­ture and the wheel­base to be short­ened. The cylin­ders now had con­ven­tional ax­ial fins while the heads had trans­verse fins across the top of the com­bus­tion cham­ber, bent into a V-shape to clear the spark plugs. Fit­ted to short, al­most straight in­let tracts the Amal GP car­bu­ret­tors shared a sin­gle float cham­ber. The Lu­cas ro­tat­ing-mag­net mag­neto was now chain driven. A new frame used the re­designed en­gine as a stress mem­ber .

The 1952 cham­pi­onship sea­son started well for AJS, with the spike­less Por­cu­pines fin­ish­ing first, sec­ond and fifth at the Swiss GP in Berne, but that suc­cess wasn’t re­peated, even when AJS re­verted to a full-cra­dle frame the fol­low­ing year. In a fi­nal at­tempt to re­gain its com­pet­i­tive edge the E95 was up­dated one last time for the 1954 sea­son, with changes to re­duce height and im­prove wind pen­e­tra­tion. The head­stock was low­ered and the forks short­ened, while the frame was also low­ered about 1½in. AJS bosses in­sisted on us­ing ‘jam­pot’ shock ab­sorbers, although none of the works rid­ers ap­pre­ci­ated the im­por­tance of pro­mot­ing the road­ster’s rear sus­pen­sion. But it was the 6½-gal­lon al­loy fuel tank that was the star of the MKII E95. With sides that reached nearly to the swingarm spin­dle, it was re­ally two pan­nier tanks with a bridge over the top. Thanks to re­cesses in the pan­niers, the tank pro­vided ef­fec­tive stream­lin­ing for the rider’s legs.

Be­cause the petrol taps were now be­low the car­bu­ret­tors, fuel flowed from the main tank to a float cham­ber mounted un­der the back of the gear­box. A pump, driven by the mag­neto drive, fed the fuel from the float cham­ber to a header tank in­side the main tank. Grav­ity then fed petrol to a pair of weirs each side of the carbs which also set their fuel level. Sur­plus fuel fell over the weirs and drained back to the float cham­ber. To prime the sys­tem, the bike had to be stood nearly ver­ti­cally on its rear wheel to get the fuel over the bar­rier be­tween the header tank and the main tank!

But de­spite its im­pres­sive looks, the E95 was no match for a Nor­ton Feath­erbed or a Gil­era Four. Rod Cole­man, Derek Far­rant and Bob Mcin­tyre were the works rid­ers for the 1954 Se­nior TT. On the first lap Far­rant slid off his E95 at a wet Quar­ter Bridge, only a mile from the start. Cole­man held fifth place for three laps but re­tired with a split fuel tank. And Mcin­tyre even­tu­ally fin­ished in 14th place. Five days later, at the Ul­ster GP, the world’s fastest road race, Cole­man fin­ished sec­ond with an av­er­age speed only 0.65s slower than Ray Amm’s win­ning Nor­ton. The Por­cu­pine was giv­ing 55bhp and 145mph, but it wasn’t enough. At the end of the year, AJS an­nounced they wouldn’t be send­ing works teams to the 1955 TT or con­test­ing in­ter­na­tional road races. The Por­cu­pine had been well and truly spiked.

‘TO PRIME THE FUEL SYS­TEM, IT HAD TO BE STOOD NEARLY VER­TI­CALLY ON ITS REAR WHEEL’

Jock West gets the first taste of rac­ing the new bike in 1947. It was rather bit­ter

Les Gra­ham at the 1949 Ul­ster GP. He won, and took the Por­cu­pine to vic­tory in the first 500cc World Cham­pi­onship

Fred Clarke, AMC de­vel­op­ment en­gi­neer, with the orig­i­nal 1947 AJS E90 ma­chine Jock West (left) on a Por­cu­pine and Bill Do­ran on an AJS 7R TT, taken in 1948

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