BSA M20 & M21
Buyers guide
WM20 | Sidevalve | War bike | 496cc | Girder fork | Telescopic fork
▲ Sump's own 1945 WM20, slow slogging fun but currently in hot demand by fans of motorcycle militaria and by general classic bikers
BSA M20 ownership tips It's not just the bike. It's the journey
£15.99 plus P&P | M-Series/M-Group sidevalves
The BSA M20 and M21 sidevalves were introduced in 1937 and owe their existence to the draughting pen of a certain Valentine (Val) Page—the same highly-talented ex-JAP and ex-Ariel designer responsible for machines such as the technically creditable (if stylistically wanting) Triumph 6/1, the redoubtable BSA Gold Star and, not least, a wide range of highly underrated Ariel singles. Discontinued in 1963 (1955 for the M20), two engine sizes were listed by the Birmingham Small Arms Company; the 496cc M20 (82mm x 94mm), and the 591cc M21 (85mm x 105mm up to 1938, and 82mm x 112mm thereafter). Both bikes started out with hand change, rigid frames and girder forks. That was pretty much the pre-war state-of-the-sidevalve-art.
▲ 500cc civilian model for 1945. Yours for £65, speedometer extra. Note the larger "panel" tank. Pre-war examples have many engine parts that are not interchangeable with post-war models.
For 1940, foot-change was standard, which suited the British Army and helped persuade the MOD to press the thumper into military service. The WM20, however, wasn’t ideally suited for warfare. It was too heavy, too slow and had poor ground clearance. But it was (small mercy) durable and easy to repair, which must have been a key factor in the Birmingham Small Arms company being awarded a contract to supply its latest product in huge numbers.
At the cessation of hostilities, no less than 124,334 of Britain’s most famous sidevalves had been flogged to the military, a good proportion of which are still giving ‘sterling service’ in various far flung corners of the empire.
Post war, these motorcycles became available once more for civilian usage. Although largely intended as sidecar hacks, M20s were initially sold without the necessary sidecar lugs (probably to dispose of surplus ‘un-lugged’ military frames). The 600cc M21, meanwhile, was always intended principally as a sidecar machine, hence its lower gearing, heavier flywheels and larger section rear tyre. Along with these new bikes came thousands of decommissioned military mounts which were practically throwaway machines in their day. And many owners did just that (or—according to popular folklore—in some instances actually buried them in their gardens. Go figure). Immediate post war models were supplied with girder forks—probably partly to use up military stocks. But by 1948, telescopic forks were fitted which necessitated a (clearance) modification to the front downtube to prevent the front mudguard (now enjoying greater travel) from slamming into the frame. This, in effect, means that frames built for girder forks aren’t really suitable for teles. Take note of this if you’re buying a box of assorted bits. An alloy cylinder head, intended to help redress the M-Series’ long-standing heat dissipation problem, was introduced for 1951. A brass spark plug insert was fitted to the head. Plunger rear suspension (for both models) arrived in 1951 (rigid frames, however, would continue alongside until 1960). A dualseat was an optional extra in 1952.
Amal carburettors, AA mounts and alternators
Throughout the production run, there were various carburettors/float chamber changes including Amal Type 76 (1937-38), Amal 276 (1939-45 - image right) and a 376 monobloc from1955 onward. But many owners simply fitted—and continue to fit—whatever worked. Tip: Try an Amal Mk1 concentric. In 1958, the M21—indefatigable, but dated—was listed as a ‘special order’ machine only. But the AA (along with similar organizations) clearly had a penchant for them because they continued buying the (special order) model until 1963 when the curtain closed forever. In 1961, the model supplemented its dynamo with an alternator (AA mounts only). Were the BSA M20 and M21 the last sidevalve motorcycles produced in Britain? Not quite. The 498cc Triumph TRW (sidevalve twin) was produced well into the 1960s for government use, and the 192cc LE Velocette (watercooled sidevalve flat twin) continued until 1970).
Girder forks, dampers and handling
Riding these machines takes a little getting used to. If you're not experienced with girder forks, they'll feel very different to teles, being heavier and a little more cumbersome—until, that is, you stop fighting them and sit back in the saddle and give them a little free play. On rough roads, girders will bounce around a fair bit and will occasionally pogo, but at no time do you ever get the feeling that you're about to be pitched off. It's pretty much the same if riding through potholes; when you see one coming, you instinctively flinch and prepare yourself for a gravel bath. But the heavyweight M-Series sidevalve just steams on and on looking for the next war to fight. Potholes, therefore, are less of a nuisance than a rippled road, which of course highlights the poor damping of girder models. You can fiddle with the (side) fork damper knob when on the move, but it's doubtful that you'll ever find a happy medium. That said, your riding comfort improves once you ride the M20 within its own terms. You'll get into a rhythm, and then things settle.
Telescopic forks and plunger frames
Tele-forked models are much more forgiving and absorb pretty much everything without fuss. Plunger frames, meanwhile, make longer journeys significantly less of a chore, especially for pillion passengers. Cornering is surprisingly good. The weight is low. The rigid frame is more predictable. And the power delivery is solid and dependable; perfect for hauling you out of tight curves. But the steering damper will probably need to be adjusted to suit your riding style and bodyweight/luggage, etc. These motorcycles will snake a little if the damper is too loose. Nothing serious, and nothing that can't be quickly remedied. In general, the engines on all models are strong and stress free. They should all start on the first or second kick and settle into a low, steady (and soft) beat. The procedure is straightforward; tickle the carb, set a little retard on the ignition, bring the piston over TDC (with or without the valve lifter) and swing through gently and firmly. And keep in mind that it's really the last few kickstart degrees that does the job, so don't ease off halfway down. Different examples, naturally enough, will have slightly different starting routine. Some want choke. Some don't. Some like a good flooding. Some like it leaner. But all examples should be easy starters, cold or hot. On the move, these sidevalves like a handful of revs, but they start to get more seriously vibratory at around 50mph. Therefore, you'll probably cruise a little under that speed; maybe a steady 45mph. You'll find that once running, the ignition can usually be fully advanced; you won't have to keep fiddling with it, unlike some other classic motorcycless—except, that is, when on a steep hill or when trickling through traffic, in which case you should retard the ignition to smooth out the power pulses. A little experimentation with the advance and retard lever will improve your control, but avoid riding with the ignition retarded under normal riding conditions; you'll burn out the exhaust valve. Gearboxes are slow and need a patient foot; tip, count a full second between changes. The clutches are light and reasonably progressive. They shouldn't snatch, so if they do you know that something needs adjusting.
The brakes are adequate at the front and a little better at the rear—except for later 600cc examples with an 8-inch front brake which is very good. Our own BSA WM20 has a brake rod conversion on the front. With this arrangement, the brake cable is shortened and stops at a cable lug near the top of the forks. From there, a rod takes over. This is useful because the standard brake actuating arm doesn't have a fine adjustment facility, while the rod conversion offers a wider range of movement. Also, you don't get any stretch in a rod as you might in a cable (not that a cable should stretch more than a few percent when the brake is full on). Regardless, the rod conversion has improved things a little. The real solution is to have the brake shoes skimmed on a lathe and to fit the softest brake compound you can get. Fuel consumption at around 40-45mpg is poor (when compared to 500cc OHV singles at 55-60mpg), and there's not a lot you can do about it except ride slowly and eke it out. Old road tests talk of M20s doing up to 70mpg. But to get those kind of figures, you've pretty much got to be pushing it down the street. Why is the fuel consumption so high? Because it's a sidevalve. It's as simple as that. The gas flow through a sidevalve engine is convoluted. At low speeds, the air/fuel mixture can find its way in and out without any major turbulence. But once the piston speed increases, you've got a storm inside that heats up the mixture and the metal and doesn't vent properly. Crudely put perhaps, but essentially that's how it works.
BSA M20 Tuning
People talk about fitting Gold Star cams and other polishing/porting tricks. But if it was that easy, the factory would have done it. Broadly speaking, there seems little point in those kind of mods unless you're planning to race one, in which case you need to plumb in an updraught carburettor (right under the inlet valve) and do some serious welding inside the head to build up material to help control the gas flow. You might also want to relocate the spark plug and raise the compression, etc. But it's simply not worth it for street use. Better to enjoy the bike within its own terms. Travelling long distance on one of these hacks isn't as bad as it might seem. Naturally you'll take longer to get anywhere, but there is some fun to be had at 45mph cruising, not least that you get to see a little more of the world instead of having it all flashing by in a blur. Upon settling in following a rebuild, these old hacks usually become wonderfully reliable, and when things do go wrong, they're generally quickly fixable at the roadside. You won't need a huge range of tools when travelling. Just make sure that they're all Whitworth, and that you've got some cable nipples, scraps of electrical wire, a spare plug, spare points, HT lead, electrical tape, bulbs, drive chain and primary chain repair kit, plus a handful of nuts and bolts of varying sizes. Better carry a tube of gasket cement too, and some fuel line and fuel line washers/corks, etc. It's not as much as it seems. In fact, everything you need will pretty much fit in the tool box, with perhaps a few extras stuffed in your jacket or shoulder bag. Electrical problems should be infrequent. The wiring loom is simple. Both models have magneto ignition, and provided the bike starts well when you set off, it should keep working well enough to at least get you home.
▲Bonhams sold this circa-1939 BSA M20 civilian example in April 2013 for just £2,760 including buyers premium. That's a true classic bike bargain. There were no documents with the lot, note. Click on the image for details, or follow this link
1. Designer Val Page (1897-1978) was also responsible for the futuristic two-stroke Ariel Leader (1958-1965). 2. The Birmingham Small Arms Company was founded in 1861 by 14 gunsmiths formed to supply munitions to the British Government. 3. An OHV B-Series 350cc or 500cc single cylinder engine will fit an M-Series machine, but you'll have to change the exhaust, fuel tank and one or two other minor parts. 4. There are two types of M20 gearboxes; early and late type (pre- and post-1949). They are interchangeable. 5. The bottom end is just about unburstable. The same basic crankcases are used in the high power DBD34 Gold Stars, and you have to work hard to blow up one of those. Parts will wear out, but the fundamental design and build is highly unlikely to give major problems on the road.
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In 1956, an 8-inch front brake was introduced for the "six-hundred" while the "five-hundred" was dropped from the range. 
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