BSA B31
Stout fellow
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.