Thunderbolts and Lightning: BSA Motorbikes

By David Porter, Customer and Technical Support at The Bonneville Shop

Table of Contents

When discussing the history of the British motorcycle industry and some of the most famous marques, like Ariel, BSA, Norton, and Triumph, you won’t get very far into the discussion without mentioning names like Val Page, Edward Turner, Bert Hopwood, Doug Hele, and others who were all contemporaries during the era of post-war England. These men were the forefathers of British motorcycle design and engineered some of the most admired and reliable machines of the day. In particular, Jack Sangster stands out as a pioneer, having founded Ariel in 1932, he employed Val Page, Bert Hopwood, and Edward Turner.

Pioneers in British Motorcycles

1930s BSA History

In 1936, Sangster purchased the financially struggling Triumph Cycles and re-established it as the Triumph Engineering Company. He then brought Turner over from Ariel, and the Speed Twin, Turner’s parallel twin cylinder engine debuted two years later, setting the standard for design and engineering. Meanwhile, Val Page and Joe Craig were busy developing a vertical twin cylinder engine at BSA. In 1938, an interesting prototype model was introduced, a 500cc overhead cam vertical twin engine housed in a BSA Silver Star frame with M-series cycle parts. Featuring an impressive large alloy rocker cover, this motorcycle was said to have 100 mph capability, but never saw production as BSA turned their attention to war time production.

1940s BSA History

Sangster sold Ariel to BSA in 1944, and in 1946, the first generation 500cc A7 BSA motorcycle was introduced, having had the basic layout of the engine credited to Val Page, although he had left BSA to return to Ariel several years earlier. Hopwood had been at Triumph, under Turner, then at Norton in the late 1940s, where he designed the 500cc Dominator engine.

1950s BSA History

He joined BSA in 1948, and by 1950 a revised A7 and new 650cc model, the A10, were introduced shortly before Sangster sold the Triumph Engineering Co. to BSA in 1951. In the meantime, from 1955 to 1961, Hopwood and a brilliant engineer named Doug Hele were working for Associated Motor Cycles, of AJS, Matchless, and Norton fame.

When the financial condition of AMC was becoming more and more worrisome, both Hopwood and Hele were recruited by Edward Turner, director of the BSA-Triumph group. Bert Hopwood was installed as Triumph Director and General manager, with Hele taking the reins of the engineering department. The overview of the nomadic nature of these legendary figures in the British motorcycle industry is requisite in understanding how the research and development evolved from pre-war designs into more modern unit-construction motorcycles in the late 1950s and into the 1960s.

1960s BSA Motorcycle History

The 650cc A65 BSA motorcycle was one of the most popular bikes in the world in the 1960s, especially in the US. When it and its little brother, the 500cc A50, first appeared on the market in 1962, the BSA A7 and A10 Golden Flash were at the end of the line. The sporting A10 Super Rocket and Rocket Gold Star were to continue in production into the 1963 model year.

The unit-construction twins were conceived by managing director Bob Fearon and chief development engineer Bert Perrigo and were based on the previous non-unit A7 and A10s, with similar architecture. Not coincidentally, the new unit-construction Triumph 650 models were also debuted for the 1963 model year. Lucas was intending to phase out magnetos and dynamos in favor of points and coil ignitions and alternators, so updates to both the BSA and Triumph big twins were implemented by the BSA-Triumph group.

The A50 and A65 shared many common parts and features, allowing for enhanced production efficiencies and reduced costs. The two engines had the same stroke of 74mm with the same cylinder spacing, in effect making the bottom ends the same. An A50 can be made into an “A58” by swapping out the pistons, cylinder barrel, pushrods, and cylinder head for the same group of parts from an A65 engine. Undoubtedly there are still some A50 “sleepers” out there that will run right along with 650cc models, having had the conversion done.

The use of a single camshaft was a hallmark of the Hopwood A7/A10 BSA motorcycle design and was implemented on the unit-construction twins, allowing for quieter geartrain operation with only a crank pinion, idler gear, and a single cam pinion. The pushrod tunnels were cast into the cylinder barrels, which helped maintain an oil-tight top end of the engine.

The updated design of the rocker towers and rocker spindles also made tappet adjustments easier, and the “comb”, as used on the A7/A10 engines was no longer needed to hold the pushrods in place when reassembling the top end. The rocker cover was now a single casting utilizing one gasket, another feature that led to ease of tappet adjustments and better oil leak prevention. There were also improvements in the primary drive, with a cast clutch sprocket and a three-row chain, for improved durability.

The transmission was now housed in a gearbox integrated with the crankcase, as one unit. In my opinion, one of the finest design features of the A50/A65 BSA motorcycle models was the “cartridge” style of transmission, where the mainshaft, layshaft, camplate, selector forks, and all of the gears (minus the high-gear) are assembled onto an access plate, which is fitted to the gearbox housing under the inner timing cover. The whole assembly is easily fit into place without much of a struggle trying to manipulate the selector forks, etc.

Another improvement to the unit-construction BSA twins was the chassis design. The lightened frame and low center of gravity of the engine unit contributed to predictable handling characteristics, and it wasn’t long before the BSA division started offering performance options to the A-series range.

BSA motorcycles were always competitive in road racing and scrambles events with their big single cylinder Goldstar models, but the new A65 twins were found to be especially suitable for flat track racing, as well as scrambles competitions.

The 650cc Hornet was offered from 1964-1967 in off-road competition guise, while the Spitfire model was the super sports road-going model, with the celebrated 190mm front brake, Amal GP carburetors, and shouldered alloy rims. Even the relatively heavy 500cc A50 was offered in competition models, such as the Cyclone and Wasp. The oversquare A65 engines responded well to performance tuning, and they were capable of revving to 7000 rpm, if the rider could withstand the vibration. Dick Mann, David Aldana, and Jim Rice are some of the more famous names associated with BSA racing success in the 1970s.

The vast majority of A-series twins were purchased for street use and touring. The A50 Royal Star was a mild-mannered single carburetor tourer. The A50 was overshadowed by Triumph’s T100 models, which were lighter and faster, so there weren’t near as many of them sold in the US, and the model was dropped from the product line after the 1970 model year.

Late 1960s -1970s BSA Motorcycle History

Meanwhile, the A65 enjoyed many years of sales success worldwide, with the two most popular models being the A65T Thunderbolt and the A65L Lightning. The Thunderbolt was a single carbureted model, aimed at touring, while the Lightning was fitted with dual carburetors and a tachometer. The A65s were attractively outfitted with plenty of chrome and priced well too, so they enjoyed a large distribution in the US and are still revered by vintage British motorcycle enthusiasts, particularly the 1967-1970 models.

The 1971 and 1972 models were a visual departure from the aesthetics of the previous models from the 1950s and 1960s. They were built around a new oil-bearing frame, painted in “dove grey”, and with the painted “bread loaf” petrol tank, alloy fork lowers, conical hubs, and updated electrical equipment. Although not universally loved by everyone, there were some useful upgrades to the engine, such as larger 5/16” UNF rocker box bolts and the cast iron oil pump body. The Triumph-style clutch actuator and 3/8” UNF cylinder base studs were other upgrades that were carried over from the 1970 models.

One other interesting model, the A70, was a stroked A65 resulting in 750cc-good for 52 horsepower and 120 MPH! Apparently, even while in dire financial straits, BSA didn’t want to be outdone by the 750cc 1970 Triumph T120RT and responded by introducing the A70L. A total of 202 units were produced, satisfying racing homologation requirements. Often carried by Trackmaster, Redline, or Champion aftermarket frames, the A70 engines are still coveted for their performance on dirt tracks to this day. Sadly, Dick Mann’s win on a BSA A75 triple at the Daytona 200 in 1971 came at a time when BSA was nearing the end of the line.

The Legacy of BSA

Today, the BSA A65 remains a popular model for both restoration and customization, and fortunately there are plenty of them still around. Parts are easy to source due to a robust aftermarket of reproduction spares. The early A65s present some challenges, as they used many parts and assemblies unique to them only, compared to the later 1960s and 70s models, which shared some parts with the Triumph 650 models.

By the time the Umberslade OIF models were introduced to the market in 1971, there was a degree of “badge engineering” in place, although the unique BSA twin engine was unmistakable visually and audibly. Having personally owned several unit BSA twins, I would recommend searching for a 1967-1970 model for restoration.

We at The Bonneville Shop have increased our commitment to stocking spares for the A50 and A65 models, and now feature Hepolite piston rings, big end rod bearings and oil pump, Kibblewhite valves and valve guides, along with many other bushings, oil seals, gaskets, seats, exhaust, wheel goods, etc. to keep your BSA twin in tip top shape and on the road.