BSA M20 & M21 Motor Mods
This saga is how I remember it. But if some of the mods are out of true date order you’ll have to bear with me. I didn’t keep notes or a diary. It wasn’t that sort of project! The idea was to prove “you can race anything” by building a racing BSA M20 to compete in the up to1948 class of the Vintage Motor Cycle Club.
The BSA M20 is best known as the WW2 British Army despatch rider bike. It was a very slightly modified version of the civilian original, a typical bike built as transport for the ordinary working man before the war. Girder forks, no rear suspension, a single cylinder sidevalve 500cc motor, power about 12bhp, top speed around 60mph, (the works roadtest bike only did 64, and the actual acceptable production range was 58 to 62). The steering wasn’t too good either! Not ideal racing material. So after some investigation I bent the term M20 to being a side-valve BSA which could have been built at the end of WW2 by an enthusiastic squaddie with an understanding C.O., using other army BSA parts eg B30 ohv 350, or as close as I could get. Then paint it army green!
So the chassis was a rigid post-war B31, (very similar to the B30), fitted with M20 girder forks. Front brake was M20 7” single leading shoe. The rear was rigid B31. Both with racing linings. Rims were 19” x WM2 alloy Dunlops. The tyres were old stock 3.25 and 3.50 racing Michelins.
A 2 gallon ZB32A BSA trials fuel tank was fitted along with an empty headlight shell which supported the front racing number plate and looked WD. A sprung saddle and sponge pillion pad were period and looked OK
Alloy mudguards from an old trials bike were found at the back of the garage, along with a 1938 oil tank which was mounted centrally.
The gearbox was close ratio plunger Gold Star with M20 end covers. This deletes the M-type casting’s oil filler. Also the inspection cover on the top was replaced with an earlier “BSA Three Speed” cover just to cause confusion.
And now the engine……
I searched out what had been done before. I read a lot about pre-war sidevalve tuning by Laurence Hartley and others. Dick Jones in Australia sent me details of a 500cc M20 built by Bill Gough which had done 105 mph on a Tasmanian beach back in the ‘50s. Bill (and others since) used a side mounted updraft inlet system, which I couldn’t use as I wanted to meet the Vintage Club rules and keep the outside of the motor looking standard(ish).
I also read up about racing Harley sidevalvers. I wrote to Dick O’Brien for help. He sent me a copy of the Harley Davidson KR tuning tricks and some advice, which basically came down to what you achieve depends on the money and work that you put in. Unfortunately this is 100% true.
The M20 was a 496cc long stroke single, bore 82mm x stroke 94mm. It had a larger brother, the M21, a 591cc longer stroke single, 82mm x 112mm. Same barrel, same conrod, different (cast iron!) flywheels and a shorter piston.
So all the garage spares were investigated. And in among the barrels was just one with a liner. BSA used barrels with and without liners. The liner was removed, leaving a rather poor quality iron barrel, with a few inclusions and some porosity. (BSA used very good barrel material without liners or ordinary cast iron barrels with very good liners). The bikes were built for durability and for profit!
The liner now removed, the remaining barrel was measured. Pistons were scrounged, measured and discarded. And then a Triumph TR7 US spec car piston arrived on the scene. With the top turned down a bit it was the right height to fit if an M21 flywheel assembly was used.. Too heavy but the right height. And at 90.3 mm slightly (!) oversize compared to the M20. So the, now linerless, barrel was over- bored to suit and it all fitted. There was a period of peering in through small holes between a fin or two at a lightbulb inside the barrel, but a genius welder at work laid arc weld around the outside of the barrel between the fins where needed. (The barrel has always weeped oil a little bit…..)
I was a bit nervous about the strength left in the now grossly overbored barrel so reverted to traditional Triumph sprint/drag trickery and put a plate across the head and bolted it down to an alloy plate screwed to the crankcase around the driveside main bearings. The timing side of the barrel is stronger with an integral valve chest. (Having seen all this my father would never stand by the lefthand side of the bike when the engine was running!).
A BSA piston pin was used with a set of special alloy top-hat piston bushes and end plugs rather than circlips.
To meet Vintage Club racing rules the inlet tract had to remain in its original place so it was just overbored to suit a 1 3/16” TT Amal. The valves stayed standard diameters but they were turned down on their top surface to make their heads as thin as possible.
Gold Star cams of many types were tried until the timing was somewhere near KR Harley. A minor snag was that they wouldn’t go round without jamming into the crankcase. So the engine was stripped yet again to allow some local clearance improvement via an electric drill and cutters. Standard M20 valve springs were used but with smaller additional springs from a pre-war ohv BSA inside them. I later found that while valve float on a sidevalve doesn’t cause the valves to hit the piston, it does close the plug gaps!
BSA used an alloy head on later models. I had the combustion chamber of one filled in and remachined it to be more like the KR. Note that another one completely refused to allow weld to stick, suggesting quite a variation of material availability just after the war.
Ignition was from the magneto bit of a magdyno. Oil was Castrol R and the fuel was ordinary leaded petrol (gasoline).
So, with everything painted army green apart from the saddle, tyres and barrel and head (matt black) off I went to the annual Vintage Club practice day at Mallory Park.
The bike had never actually run at this stage, and was an absolute pig to start for the first time, but given help from a fellow rider with a set of starting rollers under the back wheel of his van the M20 finally burst into life. There were minimal silencing rules back then, so with the 60” straight through exhaust and a bit of ignition retard it got through the noise test. Sounded quite good, well actually it sounded very good.
Went quite well too for a first attempt. I overtook a BSA A10 coming round the Devil’s Elbow (a downhill lefthander following a slow chicane and just before the start/finish line, so where people were watching!) and by the time it got into print I’d overtaken a Road Rocket, and then a Rocket 3.
But the day had shown snags too. In those days the races were started with the old run and bump start from a dead engine. And this led to the first change. Sadly the gearbox had to revert from close to standard ratios. As the bike only revved to about 5000 rpm it needed to be geared right up to get a decent top speed, and then I couldn’t bump start it with the very high bottom gear of the close ratio box.
A lot of the fixings had loosened too at Mallory due to the vibration caused by the heavy piston. At that time I was stuck with the heavy piston so everything was tightened as much as possible and lockwired.
The handling was as bad as I expected, though no worse than the M24 Gold Star I used to race. You had to concentrate and it weaved about a bit, even with the friction steering damper tightened down, although it was very good through the slow corners, (which probably helped with overtaking the A10). After the meeting I checked the fork trail and found that the combination of parts which I had used only gave 1” trail. A search of the parts bin produced some different fork links which helped a bit, but it was never very relaxing to ride!
Over the years the bike was developed. It gained twin plug ignition via a Dyna coil triggered by the contactbreaker of the now empty magdyno. One plug over each valve. The coil and total loss battery were hidden in the toolbox.
The flywheels were machined down in diameter by 6mm, not so much for inertia reduction as for crankcase oil drag clearance and a slight lessening of pumping effects. BSA fitted various breathers to M20s over the years. I used them all, and a few more besides all fitted with one-way valves.
The biggest improvement was forced on me after a one-off ride in a sprint meeting up the drive of a stately home. I got stuck in a long queue before my last run, and the bike overheated to the extent that the alloy piston plugs softened and the barrel got scored by the pin. So the poor old barrel was overbored even more (!) and got a modified Honda XBR piston as a present. Lighter. Much less vibration. Modern pistons are wonderful.
The combustion chamber was changed to give more clearance around and over the inlet valve head to get better flow through the lift range. (As per Harley). The flow bench work used water from a kitchen bin flowing through the carburettor at various valve lifts. It also used a lot of modelling clay and gave our garden a very green wet lawn in a drought season. Even after this work it still only revved to 5,100.
The fuel was changed to a 95% Methanol / 5% Acetone mix along with a teaspoon of Castrol R per gallon. Kept it a bit cooler and smelled nice! I found that with methanol the single TT float chamber was on its flow limits so added another remotely mounted float chamber ahead of the standard one. Potential flow was then 2 (Imperial) pints a minute. The M20 actually used about 1½ pints a minute on full throttle. This was probably a bit rich but it helped cool the motor.
I raced it in Vintage club races at Mallory and Cadwell. It didn’t finish every race but when it finished it was never last. It was a lot of fun and caused lots of conversations. And whatever I beat was ohv, or even better ohc, and that was reason to celebrate. I don’t know its top speed in this racing trim, but I’d guess at around 80 - 85 mph.
A sprint meeting with a flying start ¼ mile happened along. I rebuilt the M20 again for this and managed to get a fibreglass “dustbin” streamlining made from the moulds used by George Brown on his famous Vincent sprinter and record breaker, “Nero”. And because of Nero and the comparison with the M20 my bike became Claudius. You really need to read up on your Ancient Roman history to understand this one, but let’s say Emperor Nero was a fit (if mad) young guy and Emperor Claudius was an old chap who limped and stuttered…...
The girder forks were replaced by early BSA Bantam undamped telescopic forks with their suspension movement reduced to around 1”. The headstocks are different between M20/B31 frames and Bantam ones so that was more machining to change the fork’s lower yoke to fit. Unfortunately the new forks didn’t improve the trail measurement and they no longer allowed the original friction steering damper to be used, so I fitted a modern aftermarket hydraulic damper.
The Bantam forks had the same diameter fork outer tubes as Gold Star inners so a pair of old Goldie clip-ons fitted. To get them inside the streamlining they were mounted below the bottom yoke and were only 16” across. This was not ideal for steering but adrenaline gives strength…..
I managed a recorded speed of 99mph. Not quite the magic 100 dammit. Very frustrating but also very encouraging.
That Australian 105 mph speed became my target
The M20 was now just a sprinter and so more modifications followed.
I lengthened the frame by unbolting the front and rear halves of the frame and making new longer engine plates and a separate joining plate set in front of the saddle. This was, I think, within the frame modification rules as no welding was involved. Well that’s my story anyway, and no-one complained.
It all came good in October 1995 at a flying start ¼ mile on the wide, smooth surfaced runway of an old airfield at Elvington, in Yorkshire. I fitted my largest Goldie crankshaft sprocket (25 teeth) and made up a new 24 tooth final drive gearbox sprocket (from a BSA centre and a Norton Dominator outer welded together) to give higher gearing. The front tyre was at 50psi, the rear at 45. A bit different from the usual standing start pressures of 30 and 15.
A cool autumn day. No wind. A bit of mist in the air. Perfect. Nearly a mile of (slightly uphill) run-up then head down inside the fairing and look up 10 seconds later.
First run 8.28 seconds with a terminal of 111mph. Second run 8.19 seconds with a terminal of 112mph.
Not a proper pair of “each way” record runs but good enough for me.
To complete the job I ran the bike in its lightest trim without streamlining in 1996 at a standing start ¼ at the North Weald track near London. 14.95 seconds with a terminal of 85mph, giving an average speed greater than the M20’s original 60mph design top speed.
That gave me every target I’d wanted. I’d set out to build a reasonably competitive M20. It had lapped Mallory at better than its 60mph design maximum speed. It had averaged more than that for a standing start ¼ mile. It was the fastest ever sidevalve BSA. And further tuning ideas, such as bigger valves, steel flywheels, modified B type crankcases allowing the barrel and head to be “narrowed” to improve the combustion chamber volume and flow, etc, etc, (oh, I thought about it), would have started to involve much more time, work and money than the project really warrented. It was only for fun, after all. So Claudius was put back in the garage while other projects took up my time.
The streamlining and forks were robbed off it for Claudius 2, which looks similar but has a 670cc Norton Dominator engine, so I put the original girder forks back on.
Then, last year at an autojumble, I bought a genuine pre-war Brooklands Can racing silencer. (Brooklands was a huge bumpy banked speedbowl in the south of England built in 1907. The racers, bikes and cars, had to be fitted with large fishtail silencers to placate the locals). I made the mistake of resting it on Claudius. Just to see how it looked, you understand.
So later last year saw the M20 rebuilt again. This time back to a sort of “what if” specification. As in “what if racing had resumed at Brooklands after WW2” (it didn’t) and “what if there had been a clubman’s class for ex-army bikes”. So it’s in a sort of army sprinter trim. With maybe a bit of hill climbing and twisty sprinting in mind. I haven’t run it in anger yet. The latest ignition system’s giving a few problems. Wonder what Claudius’ top speed is in Brooklands trim? I mean if it’ll do say 105mph then just maybe a sidevalve BSA could have lapped Brooklands at 100. Nice thought. Hmm. Wonder if it’ll do 105….”
The exhaust pipe was as long as possible and was upswept running down the far side of the bike. I'd had previous experience of grounding downswept pipes on rigid framed racers, and once the rear wheel has been lifted off the track things get a bit too exciting!”
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