BSA A10
Pre-unit 650 Twin


THE HORSEPOWER RACE IS ON
The BSA A10 was the inevitable result of the relentless drive for more & more horsepower. Driven mostly by the performance-hungry US market (which also happened to be the largest by far), every British motorcycle maker scrambled to build a 500cc vertical twin after the war, in the mold of Edward Turner's game-changing 1938 Triumph Speed Twin.

THE A7 GIVES BIRTH TO THE A10
BSA Motorcycles followed suit with their own 500 twin, the BSA A7 in 1946. It was a fine machine and by 1950, it had proven itself to be sturdy & reliable. But, the pressure was on for more power, always more. Triumph had just punched their 500 twin out to a 650 in the 6T Thunderbird. BSA Motorcycles put their best men on it, Bert Hopwood & David Munro. Following British tradition, they started with the existing model, the 495cc BSA A7 as the template, then built off of that.

ALL NEW AND YET...THE SAME
Externally, the BSA A10 engine looked surprisingly similar to the BSA A7's engine & was laid out the same way, with one cam behind the cylinder block, driven off the right. And while they did, in fact, share some components in common, almost everything was different. They took the A7's 62mm bore & 82mm stroke & pushed them out to 70mm X 84mm for a new displacement of 646cc. The bore spacing was wider, so the crank was longer, so the crankcases were bigger & stronger. The one-piece forged steel crank benefited from BSA's advanced metals division as the rod journals were 'induction-hardened, ground & polished'. The A10 also got 2-piece lightweight alloy connecting rods made of RR56 aluminum.

A NEW HEAD
While the BSA A7 soldiered on with an iron cylinder head, the A10 got an all-new lightweight alloy head with a shallower valve angle, reworked finning for greater airflow & enhanced cooling & a single rocker box instead of two, as on the A7. This is probably the easiest & fastest way to differentiate the two. A BSA 500 A7 will have two rocker boxes & a BSA 650 A10 will have one big one.

NEW BRAKE, SAME OLD FRAMES
When the A10 debuted, it carried with it a new 8-inch SLS (Single Leading Shoe) front brake with a 1-3/8" wide hub made of Millenite (a high-grade iron alloy that again came from BSA's able metalworks). But the frames were the same as offered on the BSA A7. The new A10 could be ordered with either the older rigid frame or a plunger frame, until the new swingarm frame arrive in 1954.
ABOVE: 1958 650 BSA Golden Flash, an A10 model.

MODEL DESIGNATIONS
The first BSA A10 to be introduced in 1950 was the BSA 650 Golden Flash, & it ran through until replaced by unit construction in 1963. In 1953, BSA needed to hologate some production races, so they introduced the A10 Super Flash & killed it off in 1954, replacing it at that time with the hot new BSA Road Rocket which ran until 1957. From 1958 to 1963 BSA came out with the A10 Super Rocket complete with Amal TT racing carbs.

THE ULTIMATE A10
But the cream of the crop, the top dog, the big kahuna of not just all BSA A10's, but all BSA twins period, before or since, has to be the 1962-63 BSA Rocket Gold Star. This combined a tuned Super Rocket 650 twin engine with a single-cylinder BSA Gold Star frame, to get one of the fastest, best handling bikes of its day.
ABOVE: 1963 BSA A10 Rocket Gold Star

ANTI-CLIMAX
After helping write the book on vertical twins in the 1950's, BSA moved forward with plans to abandon it's popular pre-unit twins in favor of a whole new generation of 'modern' unit construction engines. The irony is that while these new engines certainly were better in many ways, they were never as popular as the older units they replaced & BSA began its slow slide to oblivion. The new unit construction BSA A65 650 twin & it's little brother that BSA A50 500 twin never sold in the kinds of numbers that BSA had become accustomed to in the late 1940s & throughout the 1950s.

BELOW: 1960 BSA A10 Spitfire Scrambler

BSA A10 YEAR-BY-YEAR


1951 BSA A10


1954 BSA A10


1957 BSA A10


1958 BSA A10


1959 BSA A10


1960 BSA A10


1962 BSA A10


1963 BSA A10



Check out these BSA BOOKS

BSA - THE COMPLETE STORY


BSA MOTORCYCLES: THE FINAL EVOLUTION


HISTORY OF BSA MOTORCYCLES


BSA: ILLUSTRATED MOTORCYCLE LEGENDS, by Roy Bacon


HOW TO RESTORE YOUR BSA ROCKET 3 & TRIDENT


CHILTON’S BSA MOTORCYCLE REPAIR & TUNE-UP GUIDE


BSA SINGLES 1945-1963 GOLD PORTFOLIO


BSA SINGLES 1964-1974 GOLD PORTFOLIO


BOOK OF THE BSA - OWNERS SHOP MA NUAL, PRE-WAR


BSA 350, 441 & 500 SINGLES, UNIT CONSTRUCTION 1958-1973


BSA CLASSIC MOTORCYCLES


BSA & TRIUMPH TRIPLES: GOLD PORTFOLIO 1968-1976


BSA UNIT SINGLES: COMPLETE STORY


BSA A7 & A10 TWINS, 1957-1962, by Haynes


BSA A50 & A65 TWINS: ALL MODELS


ILLUSTRATED HISTORY OF BSA MOTORCYCLES


BSA TWINS A7 & A10 1946-1962, GOLD PORTFOLIO


THE BSA GOLD STAR


MAINTENANCE MANUAL FOR BSA M20


SECOND BOOK OF THE BSA 500 & 650 TWIN 1962-1969


BSA PRE-UNIT SINGLES WORKSHOP MANUAL, by Haynes


TRIUMPH TRIDENT & BSA ROCKET 3 WORKSHOP MANUAL, by Haynes


BSA BANTAM WORKSHOP MANUAL 1948-1971, by Haynes


BSA UNIT TWINS, THE COMPLETE STORY


BOOK OF BSA SINGLES & V-TWINS 1936-1952


RUPERT RATIO UNIT SINGLE ENGINE MANUAL, BSA & TRIUMPH


RUPER RATIO, EVERYTHING BUT THE ENGINE, BSA & TRIUMPH


BSA A50 & A65 TWINS 1962-1973, Haynes Workshop Manual


BSA GOLD STAR & OTHER SINGLES, by Roy Bacon


BSA MOTORCYCLES SINCE 1950,BR>

BSA UNIT SINGLES OWNERS WORKSHOP MANUAL #127, 1958-1972, by Haynes


BSA TWINS 150 & 165 GOLD PORTFOLIO, by RM Clarke


ILLUSTRATED BSA BUYER’S GUIDE, by Roy Bacon


BSA BANTAM, by Owen Wright


BSA PRE-UNIT TWINS, by Mick Walker


BOOK OF THE BSA OHV & SV SINGLES - 250cc 1954-1970


BSA TWINS & TRIPLES, by Roy Bacon


TRIUMPH & BSA TRIPLES: THE COMPLETE STORY


BOOK OF BSA TWINS, ALL 500 & 650 1948-1962, by Floyd Clymer


BOOK OF THE BSA OHV & SV SINGLES 1945-1954: 250, 350, 500 & 600


BSA BANTAM BIBLE, ALL MODELS 1948-1971


WHERE BSAs DARE: BSA’s 1952 ISDT GOLDS & MAUDES TROPHY


ILLUSTRATED MOTORCYCLE LEGENDS: BSA, by Roy Bacon


BSA BANTAM ESSENTIAL BUYER’S GUIDE


BOOK OF THE BSA BANTAM


BSA 500 & 650 UNIT TWINS, Clymer Repair Manual


BSA COMPETITION HISTORY


BSA MOTOR CYCLING ANNUAL, 1937, Kindle






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