BSA A7

ABOVE: This 1952 BSA A7 featured plunger rear suspension, the transition between earlier rigid frames & the swing arm frames to arrive in 1954.

BIRTH OF THE BSA A7
By the onset of World War II, BSA was one of, if not THE largest motorcycle manufacturers in the world. They were making great success of their line of singles, lead by the BSA Gold Star. They'd done a few V-twins in the past, primarily for sidecar duty, but their business was singles & no one was better at it than BSA.

INSPIRED BY THE SPEED TWIN
Like every other British motorcycle company at the time, BSA Motorcycles wanted into the vertical twin business, once Triumph had launched their seminal 500cc Speed Twin in 1938. They wasted no time & jumped right in with their own Val Page design. But alas, history intervened, World War II started & all production was diverted to wartime purposes. But, as soon as the war ended, BSA resumed work in earnest. The design & basic architecture was contributed to by not only Val Page (JAP, Ariel & Triumph), but by Joe Craig (Norton & AMC), Herbert Perkins, David Munro & even Edward Turner himself (Ariel & Triumph). Numerous prototypes were built between 1938 & 1945 before production was reached.

ENGINE DESIGN
What emerged was a vertical twin cylinder engine with cast iron cylinders & head, a vertically-split aluminum crankcase, with the cam behind the cylinder block. The pushrods passed through an integral tunnel inside the cylinder block casting, to the rocker box. The crank pins were arranged inline (360-degrees) providing for one power stroke for every engine revolution. The bore & stroke were 62mm X 82mm for a displacement of 495cc. It was considered by many to be the quintessential British vertical twin of the era. It produced 26hp at 6000rpm.

INTERNALS
The crankshaft was pressed together out of 3 parts with plain big end bearings & one-piece connecting rods. The silicone alloy pistons were flat-topped with a compression ratio of 7.0:1, to accommodate low octane 'pool' petrol available the Brits at the time.

CYLINDER HEAD
BSA called their new cylinder head 'monobloc', designed to provide excellent air flow over, under, around & through all the hot parts, with large openings between the cylinders & under the rocker boxes. The widely-splayed exhaust ports & the rear pushrod tunnel aided in this.

A7 MARK 1
This first version of the BSA A7 500 twin was called the A7 Mark 1 & it ran from 1946 through the 1950 model year, when replaced in the 1951 model year by, you guessed it, the BSA A7 Mark 2.

SEMI-UNIT CONSTRUCTION
The custom at the time was to assemble the separate crankcase, primary case & gear box (all separate components) with heavy mounting plates, with slotted bolt holes that allowed the gearbox to be rotated in its mounts, as a means of adjusting the primary chain. This was crude & inconvenient (the final drive chain needed to be adjusted after the primary chain), was heavy & inherently weak. The BSA A7 featured a new design, dubbed 'semi-unit'. Here the crankcase & the gearbox were solidly mounted to one another, with the gearbox in a fixed position. Now primary chain adjustment was accomplished with a slipper-type tensioner inside the primary chain case. This not only made chain adjustment much easier, but also made for a much more rigid engine/gearbox structure, almost on par with 'unit construction' types. That gearbox was an all-new 4-speed foot-shift box that was lighter, more compact & smoother shifting than the units BSA was using on their singles.

FRAME & RUNNING GEAR
This new engine was placed into a new frame, specific to the twin. It was a rigid frame of brazed lug construction, with two front downtubes & full cradle construction. The new telescopic front forks were taken directly from the BSA Singles' parts bin, as were the front & rear brakes, both 7-inch SLS. Overall, it was a good-looking, well-put-together machine that offered decent performance & class-leading reliability & smoothness.

NOT THE FASTEST, JUST THE QUIETEST
BSA quickly garnered a reputation for building machines that weren't quite as fast as the Triumph twins, but were more oil-tight & quieter. They achieved this quietness through robust design & then-unique features like the semi-unitized crankcase & gearbox, a single cam (instead of 2 like the Triumph), timing gears (instead of chains like Ariel & Royal Enfield) & inner & outer timing cases. This reputation served BSA Motorcycles well, over the years.

PLUNGER REAR SUSPENSION
The first 'plunger' frames appeared at the Earls Court Motorcycle Show in 1948 & were first used in production on the top-of-the-line 1949 BSA 500 Star Twin. The plunger frame would spread to the entire 500 twin lineup for the 1950 model year. While the plunger was a big improvement over the antiquated rigid frame, it did offer some suspension travel, it was at best a poor compromise. Plungers were widely used by many manufacturers at the time, for the same reason that BSA chose it. It allowed them to continue to use their old rigid frames (with a few mods), rather than ditching them for an entirely new swingarm frame.

BSA A7 MARK 2
For the 1951 model year, BSA Motorcycles brought out a revised engine. Now the bore & stroke were 66mm X 72.6mm for 497cc of displacement. They were 62mm X 82mm for 495cc). Being less undersquare, the engine was more willing to rev & picked up some smoothness. The bigger bore meant shallower combustion chambers, which in turn meant a narrower valve angle, the same as the 650 BSA A10. Compression ratios rose to 6.7:1 with an option of 7.2:1. Fuel quality in Britain was improving.

OTHER IMPROVEMENTS
The BSA A7 was now sharing more parts with its big brother, the A10 including the crankshaft, the crankcase & a new alloy head. The new crank was a one-piece forged design with a bolted-on central flywheel.

SWING ARM FRAME ARRIVES
An all-new swing arm frame was introduced in 1954, along with an all-new roadster line, the BSA Shooting Star with an all-new alloy cylinder head. All 1954 BSA Shooting Stars had the new frame. The standard BSA A7 was now offered with either plunger or swingarm rear suspension; while all BSA Star Twins came with the plunger. When the Star Twin was dropped in 1955, this left the standard A7 as the only model still offering the plunger frame.

FEW CHANGES
New wheel hubs arrived in 1958. These were cast iron, full-width hubs with flanges to permit the use of straight-pull chrome-plated spokes. The standard BSA A7 got 7-inch hubs front & rear & the higher-performance Shoot Star (as well as all the 650 A10's) got an 8-inch front unit. Since the A7 weighed pretty much the same as the A10 & certainly the Shooting Star, this left it well under-braked, considering its performance.

THE END OF THE LINE
Minor detail changes continued, but few significant changes were made in these final years of A7 production, as the Small Heath factory concentrated on finishing up the new bike that would replace it: the BSA 500 A50 & the BSA 650 A10. These new unit-construction engines, while well-engineered, never sold as well or garnered the kind of respect and owner loyalty that BSA had become accustomed to with its wonderful pre-unit machines. It must have been quite a blow to take such a major technological step forward only to take a huge step backward in sales & market share. The switch to unit construction in 1962 either coincided with, or actually spelled the end of BSA as a major player in the motorcycle market. The following year, 1963, BSA dropped its legendary 500 Gold Star from the lineup, attempting to replace it with another unit-construction job, the 441 Victor. It too did poorly in the market, when compared to the model it replaced.

THE END OF AN AGE
BSA was on top of the world in the 1950s, having emerged from WWII as the preeminent motorcycle maker. Their singles dominated the marketplace & the racetrack & their twins earned a reputation of solid construction & good performance & again, this spread to the showroom. But times & tastes were changing & alas, BSA was not.


BSA A7 YEAR-BY-YEAR


Back to top - 1952 BSA A7

BSA's pre-unit 500 twin used a plunger-style rear suspension. It allowed them to only slightly modify their rigid frame.


Back to top - 1955 BSA A7

BSA's pre-unit 500 twin thunders on through the 50s, with continuous improvements. It now had a full swing arm rear suspension, greatly improving ride & handling.


COMING SOON
We're building pages as fast as we can, always trying to deliver the most accurate information and, hopefully, the best eye-popping pictures. This takes time. We will soon have every year of BSA A7 listed with its own separate page. Please keep checking back.


Check out these BSA BOOKS


HISTORY OF BSA MOTORCYCLES


GOLD STAR BUYERS COMPANION, 5th ED.


BSA PRE-UNIT TWINS


THE BSA GOLD STAR: MOTORCYCLE HISTORY


BSA PRE-UNIT TWINS


BSA MOTORCYCLES: THE FINAL EVOLUTION


BSA SINGLES RESTORATION, BY ROY BACON


BUILDING BUDGET BRITS: REFURBISHING BSA & TRIUMPH TWINS


BSA: ILLUSTRATED MOTORCYCLE LEGENDS BY ROY BACON


BSA UNIT SINGLES WORKSHOP MANUAL BY HAYNES


ILLUSTRATED HISTORY OF BSA MOTORCYCLES BY ROY BACON


BSA & TRIIUMPH TRIPLES: GOLD PORTFOLIO


BSA SINGLES 1964-1974 GOLD PORTFOLIO


BSA A50 & A65 TWINS 1962-1973 SHOP MANUAL BY HAYNES


BSA 500 & 650 TWINS: ESSENTIAL BUYER’S GUIDE


TUNE YOUR BEEZA BSA A10


BSA A7 & A10 TWINS, 1947-1962 SHOP MANUAL BY HAYNES


BSA UNIT SINGLES: THE COMPLETE STORY


BSA TWINS A50 & A65 GOLD PORTFOLIO


BOOK OF OHV & SV BSA SINGLES 1945-1954


BOOK OF OHV & SV BSA SINGLES 1954-1970


THE GOLD STAR BUYER’S COMPANION


BOOK OF THE BSA: SHOP MANUAL FOR PRE-WAR BSAs


BSA UNIT CONSTRUCTION SINGLES


MAINTENANCE MANUAL FOR THE BSA M20


HOW TO RESTORE YOUR BSA ROCKET III/TRIUMPH TRIDENT


BSA PRE-UNIT SINGLES SHOP MANUAL, 1954-1961 BY HAYNES


BSA TWINS A7 & A10 GOLD PORTFOLIO


BSA SINGLES 1945-1963 GOLD PORTOLIO


BSA TWIN RESTORATION BY ROY BACON


BSA UNIT TWINS


BSA BANTAM SHOP MANUAL, 1948-1971 BY HAYNES


BSA A50/A65 TWINS: ALL MODELS BY ROY BACON


BSA GOLD STAR & OTHER SINGLES BY ROY BACON


BSA BANTAM OWNERS MANUAL


BSA: CLASSIC MOTORCYCLES


CHILTON’S BSA MOTORCYCLE MAINTENANCE MANUAL


CLYMER’S SECOND BOOK OF BSA 500 & 650 TWINS, 1962-1969


RUPERT RATIO BSA UNIT SINGLE ENGINE MANUAL


BSA BANTAM BIBLE, 1948-1971


BSA BANTAM


BOOK OF THE BSA, ALL SINGLES & V-TWINS 1936-1952


BSA M20/21 SUPER PROFILE


BSA MOTORCYCLES 1935-1940


BSA ROCKET 3 & TRIUMPH TRIDENT 1968-1976 BY ROY BACON


BSA MOTORCYCLES SINCE 1950


BSA MOTORCYCLING ANNUAL 1937


1971-72 BSA 250-500 OWNERS MANUAL


1967 BSA BANTAM D1/D3/D5 OWNERS MANUAL


BSA TWINS & TRIPLES BY ROY BACON


HAYNES 1954-1961 BSA PRE-UNIT SINGLES SHOP MANUAL


BOOK OF BSA 500 & 650 TWINS 1948-1962 BY CLYMER


BSA: THE SCRAPBOOK SERIES


BSA BANTAM: THE ESSENTIAL BUYERS GUIDE


HAYNES 1947-1962 BSA A7 & A10 OWNERS SERVICE MANUAL BY HAYNES


BSA REPLACEMENT PARTS LIST 1971 BSA A75 ROCKET 3


BSA REPLACEMENT PARTS LIST 1968 BSA A75 ROCKET 3


BSA INSTRUCTION MANUAL: MODEL D10 BANTAM


BOOK OF THE BSA BANTAM


BSA GOLD STAR: PORTRAIT OF A MOTORCYCLING LEGEND


BSA COMPETITION HISTORY


BSA GOLD STAR SUPER PROFILE


HAYNES 1958-1972 BSA SINGLES OWNERS SERVICE MANUAL


TRIUMPH & BSA TRIPLES: THE COMPLETE STORY OF THE TRIDENT & ROCKET 3


BSA MOTORCYCLE CATALOG 1929


1949 BSA BROCHURE: WORLDS LARGEST MOTORCYCLE MANUFACTURER


BSA BANTAM, ALL MODELS BY ROY BACON


BSA GOLD STAR: MOTORCYCLE HISTORY


BSA PRE-UNIT SINGLES, B, C & M RANGES 1937-1963 BY ROY BACON


BSA UNIT SINGLES C15 TO B50, 1958-1973


BSA A7 & A10 TWINS, ALL MODELS 1946-1963 BY ROY BACON


BSA A50 & A65 TWINS, ALL MODELS 1962-1972 BY ROY BACON


ILLUSTRATED BSA BUYERS GUIDE BY ROY BACON


BSA FACTORY SERVICE SHEETS FOR THE 1950s


BSA BANTAM OWNERS POCKET BOOK


TRIUMPH TRIDENT & BSA ROCKET 3 SHOP MANUAL



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