YOUR CLASSICS
Gareth Pemberton turned what was a forgotten project into this jaunty pre-war special
From half a BSA to a cool Brooklands-style racer
I HAVE BEEN member of the Brooklands Museum Motorcycle Team for a few years and decided I needed a period (prewar) bike. My latest project, a 1931 BSA B31-2 special, is perhaps a little different to most restorations and definitely not one for the purists. Having had the opportunity to ride Perry Barwick’s Freddie Clarke replica Triumph T80 and one of Michael Digby’s 250 Triumphs, I was inspired to build my own Brooklands-style racer.
I eventually found a 1931 250cc BSA B31-2 on ebay –and surprisingly my modest maximum offer came out as the winner. I did not have ‘official purchasing approval’ at the time and, although she has been an enthusiastic supporter of my motorcycling activities, I expected a ‘Not another bike – what do you need six for?’ comment from my wife.
Prior to the purchase, I had researched some history of the model and came across a famous pre-war racer, Marjorie Cottle, who had ridden a 250cc BSA B31-2 and raced at Brooklands, so my bike became known as Marge (I hoped this would smooth things over on the domestic front).
The engine looked more or less complete and the crank felt smooth and solid when turned over by hand. The exposed valves were not moving, but the whole top end assembly looked good, so I suspected the cams may have gone AWOL. The gearbox, clutch, primary drive, magneto and carburettor were missing, and a host of other parts were needed. Speaking to a previous owner, I gathered that it had been sold on through various hands because most parts, including the tinware, are more or less unobtainable. I had an amazing stroke of luck in spotting another online advert for the correct gearbox, broken and in bits, but essentially complete. The forks were home-made replicas of a Webb type. The rear frame sections I suspected were from a different BSA model of the same period and I subsequently discovered they were actually from two different bikes. My first challenge then was to get the bike looking right and, importantly for me, rideable. First job was the frame – my key to accurate geometry is to achieve repeatability in the measurement of frame dimensions. This means taking down the measurement set-up, repositioning and repeating, sometimes five or six times. If I come in with the same numbers each time, I can be sure enough that the dimensions are correct. I aim to be
AS BOUGHT
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within 2mm, but in practice can get within 1mm. It works for me, as it did for the Egyptians when building the pyramids.
By extending the lower subframe rails, Tig-welding tube extensions with plugs into the cut tubes, and some gentle manipulation of the malleable steel drop-outs, I could obtain the correct geometry. I was going to bin the forks, but the options for girders are all expensive. I always like to build and fabricate as much of a project bike as I can, and in the end I was able to modify the home-made items. I reprofiled the heavy plate that served as lugs to replicate period forgings, fabricated a new top yoke and adjustment collar, made friction damper assemblies and machined new spindles.
By now I had rebuilt the gearbox, obtaining a few crucial parts from a UK supplier, but the kickstart assembly was damaged. I still cannot locate the kickstart ratchet pinion (BSA part number 27-4106 ) for love nor money – if any CB reader can point me in the right direction, I would be very grateful. The cover was damaged as a result of a hammer and chisel being used to dismantle it. Reluctant to chance even an expert alloy welder trying to repair such an old part, I have repaired the gasket face with a ‘cold weld’ epoxy and so far so good...
I used an expertly rebuilt Amal 276 (albeit of later manufacture and slightly oversize), secured to the cast iron cylinder head with a tapered alloy flange adapter and offset studs. The engine has a small star stamped on the rear of the crankcase, which designated a special performance build by the factory costing an extra five pounds, which seems to have included a larger inlet valve and high-compression piston. Unfortunately, the bike has neither of these parts, but it provides me with a pointer for the future.
The primary drive was put together with an old pre-unit Triumph clutch assembly and a collection of cush-drive parts picked up at the Kempton jumble. I had to modify the taper on the clutch hub using an improvised grinding attachment on my lathe and fabricated a guard from a single sheet of mild steel. Going by the principle of not damaging or reworking any of the original BSA parts, I made an exception in the case of the gearbox sprocket. It had a Reynolds tooth profile and was significantly worn. I machined off the teeth and modified a new 420 sprocket with a flanged
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not have a surplus of horsepower and I want to make the frame as rigid as possible to provide reassuring handling and keep up corner speed. I made up a simple chain tensioner, starting with some steel angle, and to be honest it is one of the easiest to adjust of any bike I’ve worked on.
The replica tank is just about leak free, and with a couple of knee pads adapted to fit and some Monza-type filler caps looks the part. The left-hand filler is to an integral total-loss oil compartment and must have been intended for a different BSA model. The rear mudguard was too wide, too large in diameter and too long, but after some cutting and rewelding it now fits the rear Dunlop K70 very nicely. The handlebars were fitted to my Norton Commando when I acquired it 42 years ago and mount new, inexpensive control levers. All the cables are made up to fit, although the clutch cable has to pass through the battery box.
Our first lockdown enabled me to complete most of the build and I managed to get the engine going more or less at the first attempt in my back garden with a push start and dropping the compression lever. After that, I took the bike to Brooklands to use the museum’s roller starter to get the idle and other adjustment set, followed by an (unplanned) inaugural run up and down the start/finish straight. The little BSA went surprisingly well and pulled strongly. My hastily-improvised hand gearlever and a dragging clutch prevented my getting out of second and front braking was non-existent, but otherwise it all worked
SEND YOUR PHOTOS TO: CLASSIC.BIKE@BAUERMEDIA.CO.UK fine. The bike later acquired a tank-mounted hand gearchange; I also carried out the usual fettling needed for a Triumph clutch and the rebuilt gearbox appears to work fine.
The only real issue I have suffered so far was a stuck exhaust valve. Removing the cylinder head it shifted with a light tap; by good fortune, and sufficient clearance to the low-compression piston, there was no other damage. It did not seem like the cast iron guide had been reamed out to the correct clearance, so this has been remedied. I intend to come up with a total-loss lubrication system from the integral oil tank for the exposed valves.
Armed with the original buff logbook, I managed to get the bike road registered with the original number, along with details of the bike’s origin, via the BSA Owners Club. Manufactured in 1931 as a BSA B31-1 side-valve it was sold through Pugh Bros in London to a Mr Tyley of Muswell Hill. The overhead-valve engine was supplied to Davies Brothers in Chester in 1930. Somewhere the two got together and my B31-2 was the result. The last stamp in the logbook was 1939 and I do not know if it was run after that.
I hope I have taken what was probably a forgotten project bike with limited potential languishing at the back of a shed and made it into a useable period special. I intend to take it for short road journeys, but mainly use it at events here and in Europe, as and when conditions permit.
For another Brooklands-style creation, albeit in a very different vein, turn to page 70