Tales of the Bantam C12, C15 & MC4
The BSA Bantam and the Harley Davidson Hummer were snatched from the Germans as 'reparations' after World War II. The Bantam started as a DKW in 1935 or 36 when it was considered modern with its flat Back to top piston, but by 1946 when BSA got it, it was already old fashioned. One could get about 4.5bhp from the 125cc if you were lucky, more often only about 3.5. The noise they made was dreadful and so was the oil they blew all over the test shop. We worked on them for a couple of years, making all sorts of experimental bits and pieces, none of which worked very well. I recall someone wanted to ride one in the International Six Days Trial and Bert Perrigo the BSA Competition manager persuaded us to build him a 'special'.
Reg Wilkes worked on it for a couple of weeks and got 5 or 6bhp over a wider range than the standard machine. Another time an Australian sent us under great secrecy his Bantam which was reputed to be very powerful and high revving and all the rest. It was highly modified and ran on alcohol blended fuel, they claimed 12bhp and had been timed to exceed 90mph We were suitably impressed and set it up on our dyno, we mixed the prescribed blend of fuel of alcohol benzole and acetone and ran it. However we could not stop it from four stroking and it would not give more than 6bhp. We were reluctant to jet it down in case we burned it up. So we gave up, deciding that the cold damp November air did not suit it, it apparently needed hot dry Aussie air.
In the development shop at BSA we used to hate the sound and sight of the Bantam. When I first went there we were responsible for Bantam development, but I disliked two strokes then and I still do. I suggested to Hopwood why didn’t he get Herman Meier who had been with on the Continental Circus trip in 1951 who was an enthusiast as I felt sure he would make them run. Finally after a lot of trouble with work permits and so on we got Herman over and he shared an office with me. He doubled the power of the Bantam in a few months. Unfortunately Herman was a temperamental type and I eventually recommended that he be moved to Redditch where the Bantams were made anyway. Here he got some good power outputs from the 125cc Bantam and developed the larger versions they eventually produced.
Occasionally a 250 C12 came into the shop, this was a simple and cheap engine, designed in 1935 as a light and inexpensive utility machine based on a Blackburn design used by Excelsior and Frances Barnett a year or earlier. It was novel in that the push rods crossed in the tunnel at the same angle as the valves.
One day the C12 production line was stopped due to repeated complaints from the Dealers of 'knock' from the engine. Mr Leake called a meeting of everyone concerned and told us to cure it or it would have to be dropped from production. A big panic ensued, several engines that were 'knockers' were brought into my shop and we found the in some cases the knock could be cured by retarding the ignition very slightly, but some of the worst examples 'knocked' regardless of timing etc. These were stripped right down for thorough examination, we noticed that the flywheels were made of cast iron whereas when the engine was first produced it had flywheels of steel stampings. We were able to get some of these and when fitted to the worst offenders it completely cured the problem. Whilst fiddling with C12 250cc we tried the alloy cylinder head from the Ariel Colt, a 200cc version of the same engine, this cylinder head cured 'knocking' and gave more power, which prompted us to make a 'deluxe' C12 which after a few quite simple modifications gave 15bhp. An engine was passed to the experimental dept for road testing, but I do not remember any conclusions being made and soon after the 015 was designed. This was another 'cheap' design based on the Triumph Terrier 150cc. In my opinion a poor thing.
The MC4 design which was very promising was essentially half an A7 twin, the better half I thought as it was more reliable than the A7. It was intended to make a 350cc version using A10 parts. This would have been helpful to the production people as so many parts would have been interchangeable. The camshaft had to be different of course for a single cylinder. The original design used a one-piece iron casting unfortunately the cast iron was not up to the job and the first one broke, possibly 'Nodular' iron would have done the trick or the larger crankpin as used later in A7 & A10 would have helped. At about this time (1952) we used our first alternators built into the engine timing case, this gave it an odd shaped bulge, they worked very well and used less horsepower than the regular generator with rotating armature. I think if it had gone into production some resistance would have been encountered from the typical motor cyclist over the odd appearance of this engine, but once they realised the advantages of the engine they would soon forget the appearance.
The MC4 gave half the power of the racing A7 but with much greater reliability. This could only be explained by the fact that the main bearings were only some three inches apart giving an inherently more rigid lower end than the A7, which suffered from crankshaft whip. One of the things we learned a lot about during the MC4 development was mechanical berathers. As designed it had a disc valve breather as on the earlier Gold Star, at high revolutions the little pen steel disc valve could not cope and as a result blew oil out in large amounts. The same problem on the early Gold Star caused several retirements in the Clubmans TT races due to loss of oil and too much oil on the rear tire became very mechanical breather conscious and tried to adapt the A7 mechanical breather to the MC4. At first it would not work satisfactorily finally we fitted copper pipe inside the crankcase, picking up breather air from a 'dry' spot in the corner of the crankcase. This was much more effective in terms of oil loss, but the crankcase pressure was disappointing until we retarded the timing of the breather valve by 70 degrees, it would then maintain a pressure inside the crankcase of between of 4 and 7 inches of water lower than atmospheric at all speeds. This kept the engine very clean, even when belting round MonthIery track at nearly 100mph.
Due to the MIRA test track not being completed we arranged to go to France and use the banked track at Monthlery for a week. We.loaded the MC4, a 350 Gold Star and two 650 A10 Police Specials into our van, which was my old race van which I had bought from Ray Amm in 1952, now fitted' with a V8 engine. Charlie Salt and I were the hobbledeboys, we drove the van via Dover and Boulogne to Paris. Arthur Lupton and Bert Hopwood drove an A40 Austin. Bill Nichols flew out to join us a little later. Strangely enough although Bill was such a dare-devil in Scrambles and on the road, he was completely over-awed by the banked track. He was unable to lap as fast as I did and I was putting on weight and my leathers were tight. Charlie being neat and slim lapped about 3mph faster than I did and I was a little faster than Bill. The demonstration of the police bikes to the National Guard was the funniest thing. The police were all Triumph enthusiasts and did not like BSAs. When our bikes lapped faster than the Police Triumphs they removed the silencers, saying "now we will go faster". We wondered whether we should remove our silencers as well and go faster still, but it was getting ridiculous. Their police model Triumph had the infamous sprung hub and at speed on the banking looked positively dangerous. It started to drizzle with rain and I thought now we will show them, our bikes had the old plunger springing, it seemed steadier than the Triumph on the banking. However they would not ride on the track in the wet. I did a couple of fast laps in the wet at about 103mph and was skating about all over the place. So we adjourned for lunch with the Distributor and his guests from the Guard Nationale. We spent the rest of our time there testing the MC4 and found the heavy front mudguards were holding our speed down to 93mph, so we removed front mudguard and tipped the headlamp to try and improve the streamlining - we could not remove jt because it contained switchgear etc. The bike was road equipped except for a megaphone in place of the silencer. This put the speed up to 96 for the lap. When timed the Gold Star and Charlie slipstreamed me on the MC4 we lapped at just on 99mph. When we switched bikes we could lap at 97. For a 250cc push rod machine which was not designed as a racer these times were very good, the engine was putting out about 24.5bhp. The racing Excelsior 250 claimed 22.5bhp on 50/50 petrol benzole. We were using pump fuel of 78 octane. However when we returned to the factory the Board of Directors decided against producing the MC4 which was a great disappointment to all of us, we who had seen its promise. A version was assembled with sheet metal covering over crankcase and gearbox, a different colour scheme and Sunbeam name on the tank, this was also turned, down. The C12 and its variations stayed in production.