BSA B50 Buyers guide

IN PRODUCTION FOR ONLY TWO YEARS, and with a bark considerably bigger than its bite, this half-litre have-a-go hero was the last great unit-single from the once mighty Birmingham Small Arms company. With its 499cc all-alloy engine, short-wheelbase and high centre of gravity, the B50 was developed for the dirt, grass and asphalt.

Thirty-five years after the last of the breed rolled off the Small Heath conveyer, it’s still capable of putting a little mud in your eye, a little grey in your beard, and a little steel in your shin too if approached without due caution.

Starting the B50

With a 10:1 compression ratio and a less than muscle-bound coil ignition, the beast was, and is, never exactly an early riser. But, given a little practice, it can be coaxed into life with the right approach and attitude.

The various starting tricks and techniques (exhaust valve-lifter, piston over top-dead-centre, mild tickle, and prayer) are both legendary and apocryphal. But once ignited, the sound is akin to that of junior members of the BSA unit-single tribe, albeit with a harder, more punchy, upbeat throb.

The lack of an air-lever means that it needs to be wet-nursed with the throttle until the alloy cylinder head reaches operating temperature, and then—snick—into first, ease off the minimal clutch and go.

The clutch is light and bites eagerly, but without aggression. The throttle screws the power from the engine with great surges of torque. If you're used to BSA pre-unit singles, you'll find yourself in a whole new single cylinder world here. This bike is anything but docile. It's got attitude and knows it.

Riding the B50

It vibrates, of course You feel that immediately. But it wants to be revved, and so you oblige. And without hesitation, the 34bhp engine dishes out the power, making for a lusty leap into what is for many riders uncharted near-hooligan territory. This isn’t, after all, a soft-boiled B31 or B33 single. It’s a hard-nosed B50 brat that makes appropriate brattish noises, most of which emanate from a top-end designed around fairly conventional pushrod, OHV rocker gear with an alloy piston bobbing up and down beneath a hemispherical combustion chamber.

It’s not the first time we’ve ridden a BSA unit single, not by a long way. But it’s the first time we’ve ridden one such as this example; a bike fitted with a 572cc big bore kit.

The extra power is immediately obvious and thumps through the bike like a pile driver. You either like that feeling or you don’t, and it would probably sort the men from the boys over any significant distance. But this is no tourer, and was never intended as such. It’s a bike intended for dirt roads, green lanes and, of course, competition. But it feels right at home on the cobbled, potholed streets of Wapping, East London boasting good-to-very-good suspension and quick steering which is light and responsive.

The two-way fork damping is adequate for pretty much anything you’re likely to encounter on the road, if not the track. And the conical hub brakes, if properly adjusted, will bring the fun to an end without reading you the riot act.

The high, wide bars don’t do you any favours at speed, of course. But you won’t want to thump along in any windblast much about 65mph. Not for long, anyway.

Also, importantly, it’s got heaps of presence and creates enough of a racket to make motorists stop mucking about with their SatNavs and mobile phones for a few precious seconds as you pass.

The gears are light and positive, and fast dabs of the clutch usually result in a slick change. But you have to remember to keep the motor on the boil and use engine braking to bring the revs down to exactly where you need them to be. There’s as much art as science in piloting a B50, but you don’t have to be a Michelangelo or an Einstein to enjoy one at a slower pace. There is some latitude here.

The speedometer over-reads by plenty—which was easy to judge by two 40mph speed cameras that, during the test, brought all the traffic down to the usual three or four miles an hour above the legal limit.

The seat height is about the same at that on a T140 Bonnie (around 30-31 inches). But the narrowness of the bike makes for a shorter path to ground. And it does feel light, which will make shorter/lighter built riders feel confident about handling one.

When launched, its oil-bearing frame was considered by many as near vertiginous. But by modern standards, it isn’t anything to lose your lunch over. If anything, it gives the bike an almost bullish feel as if leaping those Great Escape fences will work up no more sweat than passing over shadows on the asphalt.

Fuel consumption is in the mid-50s, which is less than you might expect from a 500cc BSA, and top end performance pegs out at around 90mph. But thrift and all-out speed dashes aren't what the B50 is all about. Rather, these wonderful bikes are best enjoyed in the dusty, hill-climbing, switchback, off-the-beaten track hinterland.

B50 ownership

Of the 5,700 or so that were built, survival rates are low. Which is partly why today the B50 is as rare as a priest with a clean conscience. It’s not the easiest bike to live with. And it needs a good dose of retro-engineering to get the very best from it. But you can be sure that ownership of one will, at least, never be dull.

But was it ever worth the hallowed Gold Star name that was bestowed upon it? It’s a debate that still rages.

Overall, these are great little BSA singles. You wouldn’t want to ride two-up on one—unless most of your journeys were shorter rather than longer. But as a solo mount, it’s a stylish thumper with a hidden chunk of lead in its glove.

Remember to duck.

BSA B50 tips

1. The unit single crankcases were originally designed for a 250cc engine. Accordingly, the B50 pressures beneath the piston are considerable and put a huge load on gaskets and seals. A timed breather is vital to keep the crankcase at negative, or at least neutral, pressure. Well-fitting mating surfaces are essential if you want to keep the oil in rather than out.

2. Poor handling is often attributed to the swinging arm bearings. These pivot on needle rollers and need plenty of grease to avoid seizure. Raise the bike and check for play. Replacement is straightforward.

3. The extra torque of the B50 means that clutches are prone to slippage and so need need to be kept in tip-top shape. Solutions include an extra friction plate, an extra steel plate, stronger clutch springs, and an alloy pressure plate.

4. The bore can be opened up to 90mm giving 572cc for extra stomp. You’ll also need to gas flow and fit a larger (32mm) carb.

5. Ignition coils were always weak leading to poor starting, irregular tickover, and misfires. Fit the best quality replacements you can afford along with a new condenser and quality plug lead.

6. Whilst good enough for general use, high performance B50s will require an upgraded crankpin such as Alpha. A Carillo conrod is also highly recommended.

7. Rocker boxes are prone to leakage which can be sorted by careful linishing.

8. Poor crankcase scavenging seems to affect some bikes more than others, and is said to be due sometimes to mismatched sump covers blocking oil pick-up holes.

BSA B50 SS

Dave Baron's bike

“I bought my BSA B50 SS in 1999. My son had been born a month earlier and I felt like marking the occasion and celebrating. So I got the Beeza. It came from Bill Little Motorcycles in Wiltshire priced at £2200. Bill was selling the bike for a client and making his 10% on it or whatever. Together with a mate, I went down to Wiltshire in a Transit van and brought the bike back to Ealing in West London. It was taxed and MOT’ed and … well, sorted.

For what was left of that summer, I rode it around and fitted new tyres, a spin-on oil filter kit and an electronic ignition (from OTJ motorcycles, in Kent; I'm not sure if they're trading anymore). Then I rode it some more and seized it.

The scavenge pipe wasn’t located right (or so I suspected). But whatever it was, it needed a new piston and a rebore, plus some other work.

I took it to Dave Hopwood in Gants Hill, Essex, and he unseized it and twin-plugged the head and fitted a 545cc big bore piston kit. I took it away, very satisfied, and put hundreds of miles on it—and somehow seized it again. Dave Hopwood repaired it a second time and enlarged the bore to 572cc and fitted a Carillo conrod. It also got a full bottom end rebuild. That was in 2002. Since then it’s been pretty reliable and I’ve had 7-8 years of enjoyable riding. Dave Hopwood still services it once or twice a year.

I’ve fitted minor detail modifications such as a smattering of stainless steel fittings, a pair of extended brake arms on the front, and new shocks from Falcon (which replaced the original Girlings and do a pretty good job). Also, I’ve installed a direct pressure feed to the rockers which is taken from the oil pressure switch (CCM did this back in the seventies, as far as I can recall), and there’s an overflow pipe that feeds from the oil tank in the top tube into a little shampoo bottle.

The bike hasn’t got the original petrol tank fitted, incidentally; this tank is from a 1971 B50T which shines up well and looks good. But I’ve got the original steel tank at home. It’s the same tank as fitted to the BSA B25s and Triumph T25s).

Lastly, I fitted a rev counter. The bike didn’t have one as standard. It was an optional extra.

The big bore kit gives a lot of extra power. The steering is light. The brakes are good. The suspension is excellent. The next mod will be probably be progressive fork springs on the front.

The BSA is great in the urban environment, and very good in traffic. It does vibrate a lot. In fact at 75-80mph it shakes like a dumper truck and rattles your foot off the footrest. I’ve never checked fuel economy, but the two and a bit gallon tank usually needs a refill after 80 miles or so. It’s insured as a classic, of course, and taxed as a historic vehicle, so that’s free.

Spares are easy with respect to the cycle parts. Engine parts are fairly easy too. There’s a very active website for the B50 owners club, and Mark Cook at Performance Engineering Services (PES) in the West Country sells a nice range of parts and rebuilds.

Overall, the B50 is a very underrated bike. You rarely see them on the street, or elsewhere for that matter.

Mine should have a Dove Grey frame (it was standard for 1971, and black for 1972). However, the actual colour on my bike is more silver than grey. Most of these B50s went to the United States, but there were a few earmarked for the UK.

If I were to sell it, I’d be looking for over £3000, but it’s not for sale.

Above: Sump visitor Robert Lavier, from Sweden, puts his 1971 B50 T through its paces.

BSA B50 owners advice...

“B50s are light, flickable and fun and have plenty of scope for tuning. Watch out for poor quality pattern parts, both in terms of materials and fit. You can run these bikes fairly hard, but only if you’re sure of the provenance of the parts. The original BSA spares were okay. But there’s a lot of stuff on the market that’s very dodgy. If you upgrade the ignition to a Boyer or, better still, a Rita unit, that will make life easier and give better performance. I also recommend using RD valves spring, which are light but with good poundage. A quality paper oil filter is a must. We’ve got years of experience working on and improving these bikes. I recommend both these and all the unit singles.”

- Dave Hopwood, BSA  unit single specialist

“The B50 is a good trail bike, but was never competitive for serious trials riding. It’s ideal for people looking for a mixture of street and gentle off-road work. There’s still a fair amount of NOS parts around if you look hard enough. Essential reading for any B25/B44/B50 owner is Rupert Ratio’s book which we sell for £16.50. It covers only the engine, but there’s a lot you can do as an owner/rider to improve these bikes.”

- Roger Gwynn, Draganfly Motorcycles

“I race a B50 in scrambles. It’s got a good engine and was made a little stronger than its predecessor, the B44 Victor. The B50 has stronger crankcases and a stronger cylinder head and cylinder. It was BSA’s last attempt at a Scrambler—and a good attempt that, sadly, came out too late and couldn’t compete against the Husqvarnas and CZs. It should have been around 5 years earlier to be really competitive. The weakest part is the gearbox. Ride hard and you’ll wear the gears out in a year – especially the 2nd and 3rd gears. NEB in Birmingham make a 3 speed gear cluster – and many other parts too. Talk to Nigel. He’s very knowledgeable and helpful.”

- Jim Aim, Jim Aim Motorcycles. (01787 460671), Halstead, Essex. KTM dealer.

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