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 tank transfer


BSA M20 ownership tips
Specialists and links
M-Series/M-Group sidevalves
Civilian models
Amal carburettors, AA bikes and alternators
Girder forks, dampers and handling
Telescopic forks and plungers
Brakes and fuel consumption
BSA M20 tuning

Quick review

It's not just the bike. It's the journey



BSA M20 T-shirt for classic bikers


£15.99 plus P&P

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The prices of these 500cc single-cylinder sidevalves has moved up considerably over the past ten years, but appears to have slowed in 2013. Military rides are the most sought after, largely because they appeal to two overlapping communities.

The first community is the military re-enactors who celebrate the colourful and often dramatic history of the second world war M20 (or, more accurately, BSA WM20), generally by ensuring that their restorations are period-perfect right down to the despatch rider outfits they wear and the regimental markings on the petrol tanks. These guys (as a group) know almost as much about their obsessions as the people who built them. Maybe more even.

The second group are the guys who collect all kind of military equipment from Willys Jeeps to howitzers to gun carriages to half-track vehicles. And likewise, they want all the military bells and whistles.

For a properly sorted and original WM20 (Vokes air filter, canvas bags & pannier racks, field stand, canvas grips, brass handlebar levers, etc), expect to pay anything up to around £5,000 - £6,000. But there are plenty of guys who'd want even more than that to let go of their WM20 handlebars. But generally, £3,500 - £4,500 will get you a reasonably decent, ready-to-roll bike.

Entry level prices for a tatty WM20 in running order is around £2,500 - £3,000. And anything complete (basket case or otherwise) will set you back £2,000 minimum (the breaking value is considerably higher than that). But remember, it does need to be genuine (i.e. no Indian-built repro parts).

Civilian pre-war M20s and M21s fetch up to around £5,000 for a very good one. But quality bargains very occasionally turn up significantly cheaper than that (£2,800 - £3,500).

PPost-war machines (telescopic forks, rigid or plunger frames) can be picked up for 10 to 20 percent less (£3,000 - £4,000 for a very good one, and maybe £2,000 - £2,500 for a tatty example. £1,500 post-war M20s or M21s aren't as common as they used to be, but you just might find one at that price if you hunt high and haggle hard).

Although prices have stalled a little, there's little doubt that once the economy picks up again (if it ever does), WM20s prices will rise with it, and will quite probably rise faster than a lot of other classics.

One final point worth mentioning is that WM20s (military specification models) are not particularly rare anymore. Hundreds have been repatriated, rescued and restored. In fact, infatuation with these oily relics is by no means a purely British passion. WM20s can be found worldwide without too much trouble, so in that respect, they've become victims of their own success.

BSA 500cc WM20 headlight


▲ Six volt electrics provide quaint lighting, but fortunately the 50-55mph top speed makes it unlikely that you're ever going to outrun your headlight beam. Forks are strong, but primitive. Fork damping is more theoretical than practical.

BSA 500cc WM20 fuel tank


▲ Get your knees on those rubber grips and you'll be pleasantly surprised at how you can throw one of these around bends—providing the road is reasonably smooth. But don't expect much more than around 40mpg.

Pre war 600cc  BSA M21


▲ Pre-war (1937) 600cc M21 with hand change. Note that these early engines are significantly different to war time and post war engine.

1957 600cc BSA M21. Famed as a sidecar slogger, these bikes make reasonable solo mounts with stable handling


▲ 1957 600cc M21 with telescopic forks
and plunger rear. This is the bike to choose for practical sidevalve touring, but prices are generally lower than for girder forked models, especially with regard to the war mounts in full WD livery.

BSA M20 with telescopic forks and rigid frame


▲ The 500cc BSA M20 with telescopic fork and rigid frame is a classy act with style at the back and comfort at the front. On the downside, it lacks the larger diameter front brake of the late BSA M21s which offered superior stopping power.

Want to enjoy the view from
the saddle of a WM20?

Check out this video and listen to a little jazz.

BSA WM20 Ride

Try this link too if a war story
is to your taste...

BSA WM20 War story


"The great affair is to move"



£15.99 plus P&P



BSA M20 ownership tips


1. Use a "straight" 40w mineral oil, ideally something with low (or even no) detergent.

2. Change the oil every 1500 miles, or at least
every year.

3. Exhaust valves will need lapping in every few thousand miles (not too difficult a job). Power will fall off slowly until it suddenly becomes difficult to start. If you need more than a couple of kicks to fire it up, something's wrong. Check this link for more on BSA M20 exhaust valve problems


4. Make sure the valve-lifter disengages properly to preclude holding open the exhaust valve.

5. Keep the magdyno in tip-top condition for miles of trouble free riding.

6. Don't overtighten the carb; it will distort the flange, which in turn will distort the carb body leading to an overtight slide. Also, fit a suitable heat spacer between the carb and the manifold to prevent hot starting problems.

7. Having the correct tyre pressures makes a significant difference to the riding comfort and stability on these sidevalves, especially girder forked models. Typical tyre pressures will be between 17psi and 22 psi. There is some latitude here depending on the tyre make and other factors. Check regularly.

8. If you're running an Amal Type 76 or 276 carb, there's a very small, almost hidden air bleed hole adjacent to (or at a right angle to) the pilot air screw that can become blocked thereby causing running problems. Use a piece of fine wire to clear it. But DON'T gouge out the hole. The size is critical and doesn't want enlarging. Try using a high pressure air line and/or WD40 to clear the hole before poking around with a piece of wire.

1944 WM20 Specifications

Type: Air-cooled sidevalve single
Capacity: 496cc (500cc)
Bore & Stroke: 82mm x 94mm
BHP: 13 @ 4200rpm
Compression ratio: 5:1
Transmission: 4-speed, multi-plate
dry clutch
Brakes: 7-inch drums front and rear
Electrics: 6-volt, magdyno
Front suspension: Girder
Rear suspension: Rigid
Wheels: 3.25 x 19-inch front & rear
Weight: 369lbs (dry)
Maximum speed: 55mph (approximately)

Top specialists and links


The BSA M20 website

Probably the best military motorcycle site in the world owned and managed by Henk Joore.


Owners Club


Vintage Motor Cycle Club



Ariel and BSA specialists. Hugely knowledgeable. Remanufactured and hard to get spares.


Amal Carburettors



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.


1938 500cc BSA M20


▲ 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.



Civilian BSA M20s


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


BSA M20 Amal 276 carburettorIn 1956, an 8-inch front brake was introduced for the "six-hundred" while the "five-hundred" was dropped from the range.

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.


Brakes and fuel consumption


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.


circa 1939 BSA M20


▲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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