BSA Unit-Construction Singles
BSA Unit-Construction Singles Background & History
As obscure as they may seem today, the BSA Unit-Construction Singles were in fact pivotal in the history of classic British motorcycles. BSA acquired the Triumph brand in 1951, and in 1952 Triumph released it’s new Terrier with a tidy unit-construction 150cc engine, which grew into the 200cc Tiger Cub in 1954. At the time, virtually all British motorcycles used non-unit construction, which meant that the engine, primary chain case and gear box were all separate components held together by brackets. Unit construction housed all three in one unitized casing, hence the name. It was neater, cleaner, easier to build, easier to maintain, stronger and lighter. Inspired by Triumph's foray into unit-constuction, in 1958 BSA released it’s own unit-construction single, the 250cc BSA C15 in 1958, making it the first unit-construction BSA. From these humble beginnings, this little engine that started out at 250cc, 350cc, 441cc and finally 500cc. The displacement literally doubled from about 14 years. I can’t think of another production engine that can make that claim, motorcycle or automobile. These BSA Unit-Construction Singles, in all their various sizes, filled two main roles: Off-Road Enduro/motocrosser and budget street commuter/street fighter.
At the heart of all BSA Unit-Construction Singles is 'the little engine that could'. This little jewel started out at 250cc in 1958, soon it was stretched to 350cc in 1961, stretched again to 441cc in 1966, then one final time to 500cc in 1971. It literally doubled in size in 14 years!
The ‘Unit-Construction Revolution’ happened pretty much only at BSA and Triumph (the rest of the British motorcycle industry couldn't afford it), and ran from 1958 with the 250cc BSA C15, to 1963 when BSA’s and Triumph’s 650 twins were converted to unit-construction, thus completing the process. BSA and/or Triumph no longer had any non-unit bikes.
Probably the most recognizable of all BSA Unit-Construction Singles was the BSA 441 Victor. This is a 1967 BSA 441 Victor Special, but they all look pretty much alike. Notice the old-style single-leading shoe front brake in an iron hub. Old school for 1967.
Unit-Construction Arrives at BSA
Ultimately, it had to be done. Lucas Electrics (aka: “The Prince of Darkness” because the lights don’t always work), exclusive supplier of electrical components to the entire British motorcycle industry announced that they would discontinue production of their age-old line of magnetos and dynamos that virtually every British bike relied upon for spark and lights. Starting in 1963, Lucas would be producing components for the new alternator-and coil system BSA and Triumph used this as an excuse to finally reengineer their big twins, the only engines that were still non-unit, and convert them over to unit construction, and the new electrical system. The same thing had to be done to the singles, which were still big sellers in England, less so in America. The process had been started by Triumph way back in 1954 with its line of lightweight singles that impressed BSA with the idea of unitizing its engine packages. Next was the 350cc Triumph 3T Twenty One in 1957. Next up, in 1959, was Triumph’s pre-unit 500 twins, which used to share architecture with the pre-unit 650s. The new unit-construction 500s would share architecture now with the smaller 350. Other than BSA’s Unit-Construction Singles, which the inherited from Triumph, BSA had not ventured into unit construction to the extent of Triumph. All off BSAs twins, and all of Triumph’s 650 twins were still non-unit (also called ‘pre-unit’) construction. It would have to wait until the 1963 model year when Triumph converted its line of 650s over to unit-construction, and BSA converted all of its twins. What emerged was the 500cc BSA B50 twin and the 650cc BSA A65 twin. By this time, the BSA Unit Construction Single had grown to 441cc.
The 1972 BSA Gold Star 500SS was a handsome machine. Notice the alloy conical front hub from the oil-in-frame 650s.
BSA Unit-Construction Singles Broken Down
As discussed, there are several different displacements (capacities) among BSA Unit-Construction Singles, such that it’s not easy to organize. Below is a breakdown of the various BSA Unit-Construction Singles organized by displacement, then model.
BSA C15 250cc single (67mm X 70mm), 51.2” wheelbase, 1958 to 1967
The 250cc BSA C15 was inspired by the 200cc Triumph Tiger Cub, which itself evolved from the 150cc Triumph Terrier of 1954. However, the BSA C15 was a fresh design launched in 1958 that stayed in production through the 1967 model year, and has the distinction of being BSA’s first unit-construction bike. At the same time, Britain had passed the ‘Learner Laws’ limiting new riders to bikes no larger than 250cc, so they sold well in the home market…at first…then the Japanese arrived. The C15, with its meager 15hp couldn’t compete with the likes of Honda’s 250cc CB72 with 24 hp on tap. And, like all BSA Unit-Construction Singles of the day, they looked antiquated with their cast iron barrels and brake drums. Early C15s suffered from a variety of maintenance issues which were worked out over time. The C15 was succeeded by the 250cc BSA B25 Starfire and B25 Barracuda n 1967. However, despite it's shortcomings, the amazing C15 formed the basis for all BSA unit-construction singles through the company's dissolution in 1972. It literally doubled in size in that time.
This 250cc 1961 BSA C15 was an 'enduro' version with knobby tires and high side exhaust.
BSA B25 250cc single (67mm X 70mm), 54” wheelbase, 1967 to 1971
The 250cc BSA B25 was an evolution of the 250cc C15, they shared architecture and had the same 67mm X 70mm bore and stroke. The C15 in turn evolved from Triumph’s 200cc Tiger Cub which evolved from the 150cc Triumph Terrier, quite a lineage. The BSA B25 improved on the C15 by replacing the cast iron cylinder barrel with a lightweight alloy unit, with squarish finning, and a cast-in pushrod tunnel rather than the removable part on the C15, which was one more thing that could leak oil. And they did. Valve adjustment was now done via eccentric rocker arm shafts. To improve performance over the C15 the intake tract was opened up and bigger valves were added along with a new Amal concentric carburetor. With a sports camshaft and a 10:1 compression ratio the B25 produced 26 hp at 7250 rpm. The BSA B25 gained a bad reputation due to problems with the big end of the connecting rod, a problem that BSA solved but not before many failures that hurt the reputation of the bike and of BSA. The B25 Starfire and B25 Barracuda were sportier models with parts off of the bigger bikes, like 7-inch full-width brake, 2-way dampened forks, larger fuel tanks and seats with bum stops. The B25FS Fleetstar launched in 1969 to serve fleet customers such as the police, with a lower compression ratio (8.5:1) and lower power (21 hp). The B25SS Gold Star 250 was a dirt bike/race bike. The B25T Victor Trial 250 was another dirt bike made late in the B25’s life.
250cc 1970 BSA B25S Starfire. Note the 7-inch twin leading-shoe (TLS) front brake off of the 1968-and-later Triumph & BSA 500 twins.
BSA B40 350cc single (79mm. X 70mm), 53.5” wheelbase, 1961 to 1970
In the never-ending quest for more power, BSA bored the 250cc 67mm to a whopping 79mm to create a 350 (343cc, actually). The BSA B24 made 21 hp which was enough to cruise comfortably at 50-55 mph and top out at around 75. The B50 Star was the standard street version with full-valance mudguards (fenders), fork shrouds and a headlight nacelle. The B40 Sports Star SS90 had higher compression and bigger valves and carburetor to make 24 hp. The B40 Enduro Star was a limited-edition bike launch in 1963 as a 1964 model.Built on a trials frame with a 19-inch front wheel, a low compression engine, wide ratio gearbox and while it came with lights, there was no battery. In 1964 and 65 BSA produced a special police version dubbed B40 Police. The B40 also did military duty with the B40WD as 2,000 were sold to the Royal Corps of Transport from 1966 through 1970, and the Danish Royal Army bought 1,100 B40WDs from 1967 through 1970.
350cc 1961 BSA B40 Star.
BSA B44 441cc single (79mm X 90mm), 53” wheelbase, 1966 to 1970
The 441cc BSA B44 evolved from the 350cc B40. The bore remained the same, but the stroke was lengthened by a full 20mm. The bottom end was strengthened with better main bearings and the iron barrel of the B40 was replaced with an alloy unit. The 441cc displacement was arrived at through racing experience. The B40 had been competing in Motocross and Trials evens in Europe and the UK and lessons were learned there that convinced BSA engineers that 441cc was the maximum capacity they could achieve while still being reliable. Rider Jeff Smith took several victories in 1964 and 1965 on the BSA B44. So much so that BSA released a bike based on his race bike and named it the BSA 441 Victor, which spawned variants like the Victor Special and the Victor GP. They stayed in production from 1963 through 1970, and came in two basic forms, the BSA 441 Victor which was aimed at the off-road market, and the BSA Shooting Star which was intended as an inexpensive street machine. Standard BSA 441 Victors made 28 to 30 hp, with the GP versions making 34 hp.
441cc 1969 BSA 441 Victor Special. Again, they all look the same. Note the black iron brake drum up front. This didn't play well against Japanese bikes with aluminum full-width hubs and TLS brakes. This is just one example of how the Brits fell behind.
BSA B50 500cc single (84mm X 90mm ), 54” wheelbase, 1971 to 1973
The BSA B50 was the final iteration of BSA’s Unit-Construction Singles line. The 441cc B44 was bored from 79mm to 84mm to create the extra displacement. They made 34 hp at 6200 rpm. The were strong and light. The 1971 BSA Victor 500 Trail weighed just 298 lbs. Part of the secret was a new oil-bearing frame. Most were dirt bikes, enduros or outright motocross racers in the open class. Around 5,700 B50s were built, and were certainly among the very last motorcycles BSA built before closing their doors in 1972. Some unsold 1972 bikes were retitled as 1973s. The B50 had a brilliant, albeit short, racing career in both road racing and off-road events. A BSA B50 Gold Star 500 won the 500 class at both the Thruxton 500 and the Barcelona 24-Hour, and outright won the Zolder 24-Hour race. Additionally, a BSA B50 once held the class lap record for a production bike at the Isle of Man TT. Heady stuff. Variants included the B50SS Gold Star, which was set up for road use and aimed at the US market after those back-to-back road race wins at Thruxton and Barcelona. The BSA B50T Trail catered to the fast-growing trail riding boom in the US. The BSA B50MX was an outright motocross race bike. These were some of the last bikes BSA built, and once they closed down, many were rebadged and sold as the Triumph TR5MX.
This 500cc 1971 BSA B50SS Gold Star shows how the 'Goldie' had now become the premium street bike in the B50 lineup. Note the alloy conical hubs front & rear, taken from the 1971-72 BSA and Triumph 650s.
Norton P92
Teetering on the edge of collapse, BSA attempted a merger with Norton-Villiers, but didn’t survive the transfer. What emerged was the new Norton-Villiers-Triumph company without BSA. Several ideas were tried to revive the two ailing product lines. NVT created a prototype of the Norton P92 that coupled the BSA B50 engine with the frame from the stillborn BSA Fury/Triumph Bandit 350 street bike, using Norton’s patented Isolastic Suspension to smooth things out. To comply with upcoming US requirements for left-foot shifting (all British bikes shifted on the right, as God intended) the engine had to be tilted forward in the frame to make room for a shift shaft to pass under the engine to accommodate the left-foot shifter. This model never left the prototype stage.
B100 V-Twin prototype
This was another bold experiment first tried by specialist motorcycle designer Gerald Fitzpatrick who coupled two BSA B50 motors into a 1000cc V-Twin in 1977. The frame was a slimline Featherbed Frame from the Norton Atlas. Triumph considered putting the B100 into production but couldn’t afford the retooling costs. The original B100 prototype was ridden for two years then went into deep storage. It was featured in the April 1978 issue and again in the January 1988 issue of British Bike Mechanics magazine.
This is called a 1973 BSA B50 MX, but BSA stopped building bikes in 1972. These '73s started out as 72s that didn't sell and were retitled as 1973 models. Some were rebadged as Triumphs and sold as the TR5T Trophy Trail.
BSA Unit-Construction Singles Books & Products
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Available in all sizes, in 5 colors: Black, Gray, Light Gray, Olive Drab & White. Logos are red. |
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