Speedy Beezer

B elow: Team Speed, happy in their venture. BSA’s 343cc B40 model, introduced in the early Sixties, isn’t the first choice of machine to build into a classic MXer; it’s too big for the 250 class and underpowered in the 500 class. Yes, Jeff Smith gave the 500cc riders a scare when he turned up with a factory-prepped B40 but most club riders weren’t of his calibre and none had a factory comp department behind them. The B40 did have the twin advantages of being a lot lighter than a 500 and hence a better power-to-weight ratio. A lighter bike means the available power has less to haul around and the smaller mass means it is less tiring for the rider. Such attributes were not lost on those outside the factory too.

Eric Cheney for instance had recognised these attributes very early on when building his own race bikes in the Fifties and carried on with the theme into the Sixties. In part the need to build such machines was because the main factories didn’t make a lot of competition bikes as it took resources away from profitable road machines. Even in the Sixties riders went aftermarket or foreign for machines. Naturally when the classic scene got going competitors turned to Cheney for frames and complete bikes to fill the need for competitive machinery, which is where this rather tidy looking BSA comes in.

Speaking to the bike’s owner Phil Speed and its rider Greg Speed during a break in racing at the Wasp

Sidecar Celebration last September I learnt the bike is the result of a lot of patient, occasionally frustrating and always exacting work. Says Phil with a grin: “I’ve lost count of the number of times people told me a B40-based engine wouldn’t work with a three-speed gearbox but it probably matches the number of times I was about to throw the towel in…” From this comment I gathered the 350 was a bit different from what it was when Phil bought it from the original builder Bob Kent eight years ago.

What he bought was a nicely built and well looked after machine which went well enough on dope but wasn’t anything really special. Though

NOTSOSECRET TRICK

Getting the best possible performance from a unit BSA – any British engine actually – relies on many factors and making it breathe properly is one of them. “I’ve paid a lot of attention to the breathing system and happened on a chap called Stan Millard in the US. Stan was legendary and was still using BSA B50 engines in modern flat-track racing until quite recently. He really went into the breathing aspect of his engines and I’ve tried to follow his thinking.”

Above: Feeding dope into the engine is a bigger than usual carburettor. a classic era rather than an original Sixties machine, the same care and attention goes into the modern frames built by Simon Cheney as the original ones received from his dad Eric. Based around a standard 1967 ex-WD B40 engine in a typical Sixties-period Cheney rolling chassis, there were no problems as such with the bike except it wasn’t quite fast enough and on dope some of the original BSA components were wilting a little. However the 343cc machine sufficed as it was for a while with both father and son riding it but Phil always felt the motor had more to give and the frame deserved an engine with a bit more oomph in it so set about tackling the issues.

Cheney frames are so well known sometimes their actual construction basics are glossed over as it is assumed everyone knows the details; well I didn’t and asked the question of Phil: “What Cheney frame is it?” So, for those like me who didn’t know… the frame is a single downtube Mk5 version of Eric Cheney’s design. It is made from thin wall chrome molybdenum steel tube, making it both light and tough. The top and downtubes carry the engine oil so there’s no need for an oil tank which is a useful weight saving. There is an RTS spin-on oil filter plumbed into the lubrication system which uses an off-the-shelf filter available from a well-known auto parts store in the UK. The filter ensures the engine receives only clean oil as it’s not as easy to flush out the frame tubes as 

1: Dope engines suck fuel like nobody’s business… hence twin feeds to make sure lots get to the carb.

2: Rear units are almost vertical.

3: Renthal bars are popular fitments and the risers bring them up to Greg’s height.

Right: Marzocchi forks are the starting point for the front suspension, inside are Racetech emulators and special fork springs. 1 2 3

it would have been to clean out an oil tank. Holding the rear wheel in place is a swinging arm made from the same spec material. In a departure from the norm this frame is powder coated rather than nickel plated and the swinging arm will have been coated by the time you read this. At the rear are De Roover suspension units made by Belgian suspension master Julien De Roover and their design is influenced by the car world. Phil tells me they’re fully rebuildable and tailored to Greg’s racing weight.

Front suspension is handled by 35mm Marzocchi units which sit in billet alloy yokes made by Steve Gagg and bought from fellow racer Chris Chell. Naturally enough the forks have been fully rebuilt and inside are Racetech fork emulators and coil springs matched to Greg’s weight. Using modern technology the emulators help an older suspension system work in the same ways as modern cartridge systems.

Throughout the build Phil has had the mantra ‘as light as possible’ in mind when choosing components for the machine and in effect he’s following BSA’s thinking in such a respect. The lighter the bike, the easier time it and the rider have plus with less weight to haul around there can be a broader spread of power with less emphasis on ultimate power.

With this in mind the wheels were built using Grimeca hubs from Italian machines, the rear one being from a Fantic trials bike, with alloy rims laced to them. Initially Phil wondered if the rear hub would hold up to the stresses of MX but so far it has and shows no signs of wilting. He says it has taken a lot of attention to detail to make the brakes work as well as they do.

The chassis is all well and good but needs an engine to provide the power. BSA’s ex-WD B40, introduced in 1967, enjoyed the benefits of a slight redesign that the C15/B40 unit singles had in 1966. Gone was the timing side bush main bearing and in its place was now a ball bearing and on the drive side a roller bearing was used. As standard on the WD model BSA provided a wide ratio cluster to give the model a decent top speed with a trials-like slow speed ability for those tricky bits of terrain a DR was likely to encounter. Mounted on the crank case was a cast iron barrel and an alloy cylinder head. So, not ultra-heavy but neither was it the lightest engine in the world.

Once he decided on doing the engine properly Phil set to and altered a few things which experience had shown were not up to the job. It might have been okay to use standard bits when the bike was being rebuilt at the factory each week but by 2021 a lot of development had gone on and there are now better bits available. One of the first things tackled was the 

AN EXPERIMENT

Once the realisation their Gold Star was past its scrambles best had hit BSA, the search for its replacement was on. A couple of C15S models were prepped for Jeff Smith and Arthur Lampkin and they performed well enough for the concept to advance and the lessons learnt improved the standard model. There was no way the 250 C15 could go to 500cc but the newly introduced 343cc B40 might prove a better prospect. Comp shop boss Brian Martin prepared a B40 engine, dropped it into a 250 scrambler rolling chassis and went along to Red Marley Hill Climb where he pretty much cleaned up. This and other experimental outings boosted confidence in the 343cc unit engine and proved it could work well. Even though it gave 150cc or so away in capacity in the hands of Jeff Smith it still managed to challenge the 500s despite having less power. Its advantage was that, being lighter and more manoeuvrable than the bigger bikes, the lack of power wasn’t so noticeable. A company the size of BSA has many resources available and the comp department drew on as much as they could. To increase the capacity as much as possible a bore and stroke of 79mm x 86mm giving 420cc was arrived at using as much ‘B40’ as possible… except the con rod was too short… the B40 stroke is 70mm… however the experimental department still had a couple of rods available from an engine which hadn’t made production. These rods were for an engine of 79mm x 86mm bore and stroke; even better, the forging dies were still available to make more. The result was the 420cc ‘B40’ (pictured above); naturally a lot more went into the eventual world championship-winning machine but as the main feature is about a B40-inspired bike we’ll stop there.

THE RIDER’S INPUT

If during the week the bike is Phil Speed’s domain, over the weekend at a race meet it is Greg Speed’s kingdom. The 26-year-old business development manager in the building specification world has been racing classic MX since he was 17. “I started riding trials when I was a teenager then there was a break while I did the education and exams thing then when looking about for a bike to ride, dad bought the B40 and we shared riding duty until he stepped back a bit.” So Greg, what’s it like to ride? “Can I say it’s a ‘happy’ motor, does that sound okay?” It does. “There’s power in all parts of the rev range and unlike bigger bikes it doesn’t plateau, it is actually quite like a two-stroke in the way it carries speed into the corners when I shut off,” he says. He adds: “Because the flywheels are lighter and the breathing allows it to spin without suction on the intake dragging it back, the acceleration is rather two-stroke-like too.” So fair to say you like it then? “Oh yes,” he grins. Any ambitions for the future Greg? “I’d like to try grasstrack, my dad raced on the grass and was closely involved with Dula in the Eighties.” Hmmm, so one final question Greg, favourite bike of all time, doesn’t matter if you’ve owned one or not? “Oh, very easy, HarleyDavidson’s XR750 flat-track racer!” Thanks Greg.

crankshaft, a caged roller big end type, which was replaced by the later B25 type which is a bolt-up big end with plain bearing in. Rather than use the standard B25 con rod Phil fitted a Thunder Engineering one which is stronger and a good job it is too because Phil is topping it off with an Omega piston made for BSA’s 500cc B50 range. Attractive though this is, a piston really needs a barrel to go up and down in and the all-alloy square Victor barrel which has the same bore as the B40 was fitted once it was shortened to match the stroke of the B40. Replacing the cast iron B40 barrel with an alloy one chops off a fair bit of weight but there is still a massive lump of cast iron sitting in the alloy muff… or rather it did. What’s there now is an alloy liner with a Nikasil coating, making it even lighter… shall we add that a few extra grams were saved by removing the odd fin or two?

On top of the barrel is a cylinder head with larger valves and attention to the porting and a large Dell’Orto carburettor handles sending fuel into the combustion chamber. Now, the valves have to be opened and this is done by a cam in the timing chest. BSA had lots of cams available for use in these engines and choice depended on what the bike was to be used for or what spec an owner

Above: If the drive side looks quite normal then the timing side is a bit more radical.

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3 1: Rear units are Belgian De Roover ones specially created for Greg’s race weight.

2: Barrel is a modified B44 Victor one, a very modified one.

3: The legend who was Eric Cheney and his company Cheney Racing.

required. So, Phil, what’s in there? Cue tuneless whistling and intense sky inspection… “…Er a special profile to my design…” How special? More whistling and sky inspection. After some silence he added “…All will be revealed when Greg retires from racing…” Okay, let’s move on then…

Changing gear in a scramble is time-consuming and can let the engine go off the boil so on bigger bikes a three-speed gear cluster is often used. Bigger engines are generally more flexible but Phil didn’t see why it couldn’t work on a now 388cc B40. Lots of other people could see why it wouldn’t work and he was told by any number of respected engine builders he was on a hiding to nothing. “This was the point when I almost gave up,” he admits, “but I persevered. The problem is having a set of ratios with a first gear which will allow the rider to gate quickly without the top gear running out of steam halfway along the back straight.” This needed a lot of experimentation and the difference between spot on and way off was as close as one tooth on a sprocket. “Everything needs to be spot on,” Phil says, “from the cam and ignition timing to the carb settings are all vital.”

Connecting the engine to the gearbox is an NEB clutch and a Simplex primary drive. The NEB name is well known in the off-road world and initially was associated with the speedway clutch they developed. The MX clutch is based on this hard used, often abused unit and holds up very well.

Speaking of the ignition, this system is based on a Pazon kit but modified to suit the bike and rider requirements. It is a total loss system and has a twin coil, twin plug action and is fired by a radio control car battery. “A lot of people told us the twin plug thing wasn’t necessary but it just makes it so much easier to start and while other people are kicking away trying to fire up Greg is away after one kick.” Along the way Phil has made sure the bike is as light as possible by using titanium for things like wheel spindles and aluminium fasteners to hold the cases together; if a bolt has to be heavy steel then a hole is bored through it.

As the tech talk wound down I asked what he was running on: “Still dope,” he said. So, you’re on castor oil then? “No, fully synthetic, the problems people have with dope mixing in the oil come when the bike isn’t easy to start and excessive carb flooding contaminates the oil – so far we’ve not had any problems.” Watching Greg out on the track it was clear he was putting all his dad’s work to good use.