BSA B31

Stout fel­low

Old Bike Australasia - - CON­TENTS - Story Peter Laverty Pho­tos Wally Cox, Chas Rice, OBA archives

There was not much to get ex­cited about in Bri­tain in 1945, apart from win­ning the war that is. The coun­try was in eco­nomic ruin, thanks to the war ef­fort. But there was a strong de­sire to get on with life, how­ever fru­gal it may have been, and to switch fac­to­ries back to civil­ian pro­duc­tion af­ter six years of mil­i­tary en­deav­ours.

BSA, at the time the largest mo­tor­cy­cle com­pany in Bri­tain, got the ball rolling in Au­gust 1945 with the 350cc B31, its first all-new post war de­sign, which ad­mit­tedly, in­cor­po­rated more than a few items to the pre-war range, in­clud­ing the M23 500cc Sil­ver Star- in­spired crankcases. The B31 also owed much to the 1940 model B29, but fea­tured a re­designed top end with iron bar­rel and head which was a vis­ual dif­fer­ence from the pre-war OHV sin­gles. Up front sat a tele­scopic fork, a first for BSA. But be­fore we look more closely at the B31, it is handy to look at where it came from in the lin­eage of BSA pushrod sin­gles. The very first mo­tor­cy­cle de­signed and built en­tirely by BSA ap­peared in 1910 – a fairly straight­for­ward 500cc (ac­tu­ally 499cc) side-valve sin­gle with an up­right cylin­der, fin­ished in the green hue that was to be­come syn­ony­mous with the com­pany’s prod­ucts. The 500 was soon joined by a sim­i­lar 557cc ver­sion, both sin­gle-speed belt drive. A gear­box soon came along for the larger model. The ‘twen­ties saw “Slop­ers” take favour be­fore the new range of up­right sin­gles de­signed by Val Page, in 250cc, 350cc and 500cc ca­pac­i­ties, ar­rived. These new en­gines, were mar­keted as their sports bikes and the high per­for­mance Em­pire Stars and Sil­ver Stars, were much sportier than their pre­de­ces­sors, and with tun­ing could pro­duce an im­pres­sive turn of speed. This was ev­i­denced not only at places like Brook­lands, where Bert Per­rigo was the BSA ace, but in the colonies, no­tably in the hands of Harry Hin­ton and Eric McPher­son, who rode the range to nu­mer­ous vic­to­ries, of­ten de­feat­ing the over­head camshaft op­po­si­tion. The ul­ti­mate ver­sion of Page’s de­sign was the leg­endary Gold Star, first seen in 1938 and named in hon­our of Wal Han­d­ley’s achieve­ment at Brook­lands when he was awarded the ACU Gold Star for his race av­er­age of 102.27 mph.

The afore­men­tioned B29, of­fi­cially ti­tled the 350cc Sil­ver Sports Model B29 sched­uled for 1940 was a very short-lived model, at least in civil­ian form. As ne­ces­sity de­manded, the ma­jor­ity of the pro­duc­tion run ap­peared in mil­i­tary guise as ei­ther the WB30 or WD30. The B29’s main stand­out fea­ture was in the cylin­der head, de­scribed by BSA thus: “Rigid­ity of en­gine assem­bly, cou­pled with new de­sign of over­head valve gear em­body­ing to­tally en­closed du­plex hair­pin valve springs, gives this new BSA a per­for­mance which is un­sur­passed in its class. It is a model about which any ex­pe­ri­enced motor cy­clist will be re­ally en­thu­si­as­tic, and its power out­put through­out the en­tire speed range is such that in com­bines a re­mark­able de­gree of flex­i­bil­ity with a re­ally thrilling max­i­mum speed.” The bot­tom end of the B29 en­gine mir­rored the bot­tom end of the 1939 Sil­ver Star with its two roller mains one ball race and one out­rig­ger bush on the tim­ing side. Sadly, few were able to ex­pe­ri­ence the B29’s fea­tures be­fore Bri­tain’s at­ten­tion turned to keep­ing Ger­many at bay. How­ever once this was achieved, the B31 of 1945 (mar­keted in 1946 as the XB31) was a very tidy de­sign, with the sig­na­ture ta­pered cast al­loy pushrod case, with tap­pet ad­just­ment at the bot­tom, and an ex­tremely ro­bust bot­tom end. The en­gine stuck to the tra­di­tional BSA 350cc di­men­sions of 71mm x 88mm bore and stroke, but un­like the B29, used dou­ble coil valve springs in­stead of hair­pins. The valves were cov­ered by neat al­loy cast­ings fas­tened to the iron cylin­der head by four screws, with the ex­haust valve cover con­tain­ing the valve lifter. On the right side of the head was a small plate also at­tached by screws which per­mit­ted ac­cess to the push rods and rock­ers to as­sist the assem­bly process. An Amal Type 6 1 1/16” car­bu­ret­tor (later Type 29) sup­plied the mix­ture, and sparks came from a Lu­cas magneto, en­cased in the lower sec­tion of the Lu­cas Mag­dyno unit. Apart from the new tele­scopic front fork, the re­main­der of the B31 was very sim­i­lar to the pre­war mod­els. The three-gal­lon fuel tank con­tained the speedo, which was driven by a ca­ble from the gear­box. The gear­box it­self was the sub­ject of a patent for the pos­i­tive-stop mech­a­nism. A sec­ond patent was filed for the quickly-de­tach­able rear wheel, in which the sprocket and brake drum re­mained in situ while the hub con­nected via a splined which matched to an in­ter­nal spline inside the drum. On the XB31, the rear hub was a con­ven­tional riv­eted up spool type. A third patent was filed in 1946 (and came out later) for what be­came known as the ‘crin­kle’ rear hub, which per­mit­ted straight-pull spokes and be­came highly sought (on mo­tor­cy­cles other than BSAs) for mo­tocross work. Al­though of a very sim­ple de­sign, the tele­scopic forks worked well and re­mained in the BSA range on var­i­ous mod­els for many years. The forks were com­pletely un-damped, but the ba­sic de­sign was later up­graded with im­prove­ments that in­cluded an ex­tra in­ter­nal spring and valv­ing to pro­vide pro­gres­sive com­pres­sion and re­bound damp­ing. Early in 1946 a com­pe­ti­tion ver­sion of the B31 was an­nounced – the B32. Al­though it used mainly B31 parts, the new sports model had lighter chrome plated mud­guards, lower gear­ing for mo­tocross and tri­als, a smaller fuel tank with the speedo mounted above the head­light, a high-level ex­haust si­lencer and a shield for the crankcases. A sep­a­rate magneto with a bat­tery light­ing kit was spec­i­fied as an op­tion to the stan­dard Mag­dyno set up. In 1947 a 500cc ver­sion, the B33 ap­peared, with the bore en­larged from 71mm to 85mm, the main vis­ual dif­fer­ence be­ing a larger 3.50 x 19 rear tyre and a Type 29 Amal car­bu­ret­tor. Both the 350 and 500 were of­fered with plunger rear sus­pen­sion for 1949. The B31, and the first of the B33s, were rigid frame mod­els, with a plunger rear end of­fered for both mod­els from 1949. The new swing­ing arm frame was avail­able from 1954, but ini­tially for ex­port only.

With de­tail changes, such as full-width al­loy hubs, the re­place­ment of the Mag­dyno by an al­ter­na­tor lo­cated in the pri­mary chain­case with coil ig­ni­tion and the con­tact breaker where the magneto for­merly sat, re­vised mud­guards and head­lamp cowl, the B31 sol­diered on un­til late in 1959, when it qui­etly dis­ap­peared from the model list­ing. The B33 re­mained in the cat­a­logue for 1960 but its days too were num­bered and within a year it was also dropped. Our fea­tured B31 is a 1952 model, be­long­ing to Alan Phillips, who ac­quired it as a bas­ket case al­most 40 years ago. The YB31 en­gine num­ber iden­ti­fies it as from the sec­ond de­sign fol­low­ing the XB31 orig­i­nal and pre­ced­ing the ZB31 ver­sion. The YB31 used a more sub­stan­tial frame than the XB, mak­ing

it slightly heav­ier. The speedo was also moved out of the fuel tank and mounted above the top fork yoke. In 1981 it was sub­jected to a full restora­tion, with Alan do­ing most of the work him­self, with the ex­cep­tion of the paint­ing, which was un­der­taken by magneto ex­pert Peter Scott. That’s 38 years ago and the paint still looks as good as new. Since then the B31 has been on a few ral­lies and plods along hap­pily, al­though Alan says the M20-de­rived gear­box is ‘pretty av­er­age’. In its year of man­u­fac­ture, this B31 was of­fered with a re­designed, rub­ber-mounted petrol tank, which was fin­ished stan­dard in a sin­gle colour (usu­ally green), or as an op­tion with chrome tank pan­els, as on Alan’s bike. Chrome be­ing in short sup­ply at the time, most left the fac­tory fully painted, in­clud­ing sil­ver painted wheel rims, al­though the op­tional chrome wheel rims fea­tured green cen­tres with gold pin stripes. A speedo was still listed as an op­tion, along with the Dual Seat, a tyre pump and a tool kit. Marque en­thu­si­ast Doug Fraser says, “The Em­pire Star of 1937 was the first BSA to fea­ture full en­clo­sure of the valve gear, with the mag­dyno moved from the front of the en­gine to the rear. The 1939 500cc model range com­prised the en­try level M22 (avail­able in ei­ther sin­gle or twin ex­haust port), the Sil­ver Star M23, and the M24 Gold Star. The XB31 was the light­est of the post-war pre-unit sin­gles, a fac­tor in its spir­ited per­for­mance. It also re­sponded eas­ily to tun­ing, with the var­i­ous Gold Star com­po­nents fit­ting straight in”. Out here in the colonies, the iron-en­gined B31s and B33s did ster­ling ser­vices not just as util­i­tar­ian and semi-sport­ing trans­port, but on the race tracks. A home-brewed B31-en­gined spe­cial took a very young Kel Car­ruthers to his ini­tial road rac­ing suc­cesses, while Herb Jef­fer­son on his ratty but rapid B31 scooped sev­eral Aus­tralian Short Cir­cuit (Dirt Track) ti­tles be­fore the en­gine was fit­ted into a Hagon frame and con­tin­ued its win­ning streak. In the Ju­nior and Se­nior Club­men’s classes, which at­tracted huge en­tries at road race meet­ings and es­pe­cially at Bathurst, the iron BSAs were hard to beat. Quite a Jekyll and Hyde per­son­al­ity for a mo­tor­cy­cle that was oth­er­wise a ro­bust and re­li­able util­i­tar­ian mo­tor­cy­cle that re­mained in the BSA range for 15 years.

Peter Wil­liams’ 1938 M22 with twin-port en­gine and high level ex­haust pipes at the 2019 Bathurst Easter Rally.

ABOVE AND RIGHT Bruce Turner’s 1938 sin­gle­port M22 at the 2019 Bathurst Easter rally. The M22 was the en­try level OHV 500 for 1938 and is fit­ted with a quickly de­tach­able rear wheel.

The short-lived B29, listed for 1940 pro­duc­tion but made in very small num­bers due to the war ef­fort. ABOVE B29 fly­wheel assem­bly sup­ported by four roller bear­ings. RIGHT B29 cylin­der head with du­plex hair­pin valve springs. BE­LOW LEFT NSW BSA dis­trib­u­tor Ben­nett & Wood brochure from 1947 show­ing the B31, with a list price of £219/12/11 ($439.00) in­clud­ing Sales tax.

De­tail draw­ing of the BSA tele­scopic fork in­tro­duced with the B31 in 1945.

Vic­to­rian star Ron Miles, who went on to a trag­i­cally short in­ter­na­tional ca­reer, pic­tured at Dar­ley in the early ‘fifties with his B31.

ABOVE ‘Six­ties Dirt Track hero Herb Jef­fer­son, pho­tographed by Wally Cox at Ama­roo Park Short Cir­cuit His B31 was no thing of beauty, but it sure was fast, es­pe­cially com­bined with Herb’s leg­endary tal­ent. LEFT Three weeks af­ter his 18th birth­day on 29th Jan­uary 1956, Kel Car­ruthers com­pet­ing at the In­ter­na­tional meet­ing at Ban­di­ana, Vic­to­ria on the B31-en­gined spe­cial (with a Royal En­field frame) built by his fa­ther Jack. The pre­vi­ous Easter, Kel had made a dream de­but at Bathurst, win­ning the Ju­nior B grade race on the same ma­chine.

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