BSA/TRI­UMPH 750-3 – THE ME­CHAN­I­CALS

Classic Racer - - IN DETAIL -

In de­vel­op­ing the ohvt 150 in-line three­cylin­der en­gine for its de­but sea­son in AMA road rac­ing in 1970, Doug Hele and his men in the Meri­den fac­tory race depart­ment drew on their valu­able ex­pe­ri­ence gained work­ing on the com­pany’ s 500 cc twin-cylin­der en­gine, which w had twice won the Day­tona 200-miler in n 1966 and 1967, as well as given a good ac­c­count of it­self in 500GP races. With the stock 67 x 70mm 741cc triple es ssen­tially com­pris­ing an ex­tra cylin­der hung on the side of tri­umph’ s twin-cylin­der ohv mo­tor, m the po­ten­tial for cross­over en­gi­neer­ing was w ob­vi­ous. Firstly, they maxed out the stock 741cc en­gine to the 765cc ca­pac­ity per­mit­ted un­der AMA rules via the max­i­mum al­lowed + 0.040- 0 inch over­bore which in the­ory al­lowed dam­aged cylin­ders to be bored out and re e- used.this en­tailed hom­ing the austenitic ca ast iron lin­ers to ac­cept spe­cially made high-coom­pres­sion 11:1 cast pis­tons for di­men­sions off 68x70 mm, mounted on stock con rods which were pol­ished and matched. th­ese were fit­ted to a care­fully light­ened stock 120° crankshaft, with triplex chain pri­mary drive re­tained, matched to a five-speed close-ra­tio Quaife gear­box re­plac­ing the stock four-speeder, with a light­ened sin­gle-plate Borg & Beck di­aphragm clutch. TH6 cams first de­vel­oped in 1968 for the 500GP twin were used with larger-ra­dius lifters, with stock cylin­der heads fit­ted with tougher valve seats care­fully ported, and the com­bus­tion cham­ber vol­umes mea­sured and matched. Three 13⁄ 16in (30mm) smooth-bore Amal GP car­bu­ret­tors with a sin­gle sep­a­rate float cham­ber feed­ing all three carbs were ini­tially cho­sen for greater power at full throt­tle, such as wide open on the bank­ing. Ig­ni­tion was sup­plied via a spe­cial Lu­cas en­ergy trans­fer sys­tem,with an al­ter­na­tor feed­ing re­mote con­tact break­ers with­out the need for a heavy wet-cell bat­tery, and with the three coils mounted in a sep­a­rate car­rier ad­ja­cent to the tim­ing chest.the 3-1 ex­haust

sys­tem em­ployed a long mega­phone mea­sur­ing four inches / 100mm wide at the exit, a sys­tem de­signed by Hele him­self who fur­ther re­fined this on the dyno with a gain of 4-5bhp over three sep­a­rate pipes.the re­sul­tant en­gine de­liv­ered 81bhp at the crankshaft, at 8200rpm. Nearby chas­sis fab­ri­ca­tor Rob North was re­spon­si­ble for de­sign­ing and man­u­fac­tur­ing the chas­sis, based on a de­sign he had al­ready cre­ated in con­junc­tion with­tri­umph tester Per­cy­tait.the re­sul­tant bronze-welded du­plex frame in Ac­cles & Pol­lockt45 chrome-moly steel tub­ing car­ried a mod­i­fiedtri­umph 15⁄ 16th inch (33.3 mm) tele­scopic fork with welded-on brake caliper plates. Owing to a mis­take with a pro­to­type chas­sis com­po­nent that had been sup­plied to him in­ad­ver­tently, the first-se­ries 1970 North frames had a tighter 26º head an­gle, later (for 1971) kicked out to 28º with 121mm / 4.75in of trail. Cou­pled with the ta­per-sec­tion tubu­lar steel swingarm car­ry­ing twin Gir­ling shocks, this re­sulted in a 1450mm / 57in wheel­base and a 50/50% dis­po­si­tion of the 180kg / 396lb weight, mea­sured with oil, but no fuel. For 1970 the bikes had a 250mm four lead­ing-shoe Fon­tana front drum brake cast in mag­ne­sium, with a sin­gle 10in / 254mm cast iron rear disc sourced from AP Lock­heed, which tran­spired to be atri­umph Her­ald car com­po­nent, suit­ably ma­chined down! For 1971 though, the Fon­tana drum was re­placed by two more Lock­heed disc brakes, af­ter North re­designed the chas­sis to in­cor­po­rate the cor­rect front end geom­e­try, and at the same time low­ered the steer­ing head 2in / 50mm for a bet­ter weight dis­tri­bu­tion as well as re­duced frontal area for in­creased top speed, and also widened the fork to give space for fit­ting the disc brakes. The re­sul­tant chas­sis was also lighter than be­fore, scal­ing 35lb / 16kg in bare me­tal. Ini­tial at­tempts to run plasma-sprayed alu­minium discs for re­duced un­sprung weight were aborted af­ter they ex­panded un­der heavy brak­ing of the kind needed at Day­tona from ab­so­lute top speed down to sec­ond gear twice per lap at the chi­cane and turn one, re­sult­ing in the brakes lock­ing. Af­ter Hail­wood was lucky to sur­vive a high­speed crash caused by this, they were re­placed by cast iron ro­tors sim­i­lar to what had been used at the rear of the 1970 bikes.th­ese the first fac­tory road rac­ers to use disc brakes all around. For 1971, be­sides the brake changes and the new Low­boy frames with the oil cooler moved to the front of the new Let­ter­box fair­ing, other im­prove­ments in­cluded the drive to the points cam now be­ing taken from a quill drive whose in­board end was pegged in­side the hol­low ex­haust camshaft.this fea­ture ab­sorbed tor­sional vi­bra­tion and flex­ing, giv­ing more con­sis­tent ig­ni­tion tim­ing, es­pe­cially when used with US­made Bendix points which were more re­li­able than the Lu­cas al­ter­na­tive. Power was in­creased to 84bhp at 8500rpm (again at the crankshaft) with a newth13 in­let camshaft, squish pis­tons and suit­ably mod­i­fied heads. Car­bu­re­tors were now 30mm Amal Mk.1 Con­centrics for a smoother power de­liv­ery, while mag­ne­sium pri­mary cases and wheel hubs to­gether with ti­ta­nium fas­ten­ers re­duced weight.ti­ta­nium was not le­gal for axles, but with iron plugs at each end, they passed AMA in­spec­tion, which con­sisted of pok­ing a mag­net at one end of an axle! Fur­ther weight re­duc­tion came from plas­masprayed alu­minum-al­loy disc brakes, mak­ing th­ese the first fac­tory bikes to use disc brakes all around.

Right: Mike Hail­wood on a Low­boy chats to jim Rice on a High­boy. It’s 1971 at Day­tona. Left: L-R Don Emde, Dick Mann, Dave Al­dana and Jim Rice on BSA-3S in 1973.

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