As described, the story of AJS begins with Joe Stevens, father of Harry, George, Albert John ('Jack'), and Joe Stevens Junior, an engineer who owned the Stevens Screw Company Ltd, in Wednesfield, near Wolverhampton. The business already had a reputation for quality engineering and as his sons joined the business and Harry developed an interest in petrol engines, the sons turned their attention to road going vehicles. They produced their first motorcycle in 1897, using a Mitchell single-cylinder four stroke engine imported from the USA.
Before long, the brothers began making engines, starting off with a better-built version of the Mitchell but soon developing their own designs, which were sold as proprietary engines to other manufacturers, including The Wearwell Motor Carriage Company of Wolverhampton, producers of the 'Wolf' with whom AJS entered into a contract around 1901 to supply engines initially to Wearwell's factory, but later bicycles were delivered to the AJS factory and engines fitted on site.
A further contract followed to produce engines for Clynos, but following the demise of Wearwell in around 1909, the brothers decided they should produce their own machine and named their motorcycles using the initials of the only brother to have two christian names , Albert John, and so AJS was born.
Harry Stevens designed the first two AJS motorcycles, models 'A' and 'B' and production of the first models was completed in August 1910 and were exhibited at Olympia in November of that year. In 1911 it was decided to enter the model 'B' in the Isle of Man TT races. Jack Stevens came 16th on AJS's official entry, one place behind private owner J.D. Corke on an identical machine. AJS did not compete in the 1912 TT as it was busy satisfying the demand for its products, but was 10th in the 1913 Junior. By 1914 the AJS motorcycle had grown to 349 cc, with 4 speed gears and chain final drive, but the Junior limit had been raised to 350 cc and AJS won 1st, 2nd, 3rd, 4th and 6th place in the Junior race that year.
In 1914 the company was reconstituted as A.J. Stevens (1914) Ltd, and to meet increased demand AJS moved to a new factory in Graiseley House, a short distance south of the Retreat Street premises, which became the company's office and repair department. The 349 cc machine (known as the 2 3⁄4 hp) was most in demand but the company also produced an 800 cc (6 hp) V-twin.
On 3 November 1916, the Ministry of Munitions prohibited the production of non-military motorcycles, and AJS went over to manufacturing munitions, but in early 1917 the Ministry received an order from Russia for military vehicles, and AJS was given a contract to produce part of the order with its AJS Model D machine.
This Ministry of Munitions restrictions were lifted in January 1919 and production of a much improved 350 resumed in 1920. The side-valve engine was replaced by a new overhead-valve design. It also had internal expanding brakes and chain primary drive. Cyril Williams won the first post war 1920 Isle of Man TT Junior race on his 350. AJS also took the first four places in the 1921 Isle of Man TT, and Howard R Davies bettered his 2nd place in the Junior by winning the Senior on the same 350 cc AJS. This was the first time a 350 had won the 500 cc Senior TT race. In 1922 Manxman Tom Sheard won the Junior on an AJS, with G Grinton, also on an AJS, taking 2nd.
The 1922 machine was to become known as the 'Big Port' because of its large-diameter exhaust port and pipe (initially 1 5⁄8 inches, but changed in successive years). The OHV 350 would be the mainstay of the company's racing efforts until 1927. It was first offered to the public in 1923 and became AJS's most popular
For example, in 1929 the AJS range consisted of:
- M1 Deluxe 996 cc side-valve V-twin - £76. 10s, 0d
- M2 Standard 996 cc side-valve V-twin - £66. 0s. 0d
- M3 Deluxe Touring 349 cc side-valve single - £48. 10s. 0d
- M4 Deluxe Sporting 349 cc side-valve single - £48. 10s. 0d
- M5 Standard Sporting 349 cc side-valve single - £45. 0s 0d
- M6 349 cc overhead-valve single - £54.10s. 0d (twin port), £52. 0s. 0d (single port)
- MR6 Special
Sports349 cc overhead-valve single - £62. 0s 0d
- M7 349 cc overhead-camshaft single - £62. 0s. 0d
- M8 498 cc overhead-valve single - £62. 0s. 0d (twin port), £59. 10s. 0d (single port)
- MR8 Special
Sports498 cc overhead-valve single - £72 0s. 0d
- M9 Deluxe Touring 498 cc side-valve - £54. 0s 0d
- M10 498 cc overhead-camshaft single - £72. 0s. 0d
- M12 Lightweight 248 cc side-valve single - £39, 7s. 6d
Several of these were intended to pull one of the 12 AJS sidecars produced by the company, including sports, touring and commercial models.
By 1927, it was clear that push-rod overhead-valve design was becoming dated so AJS introduced two new chain-driven overhead-camshaft racing models, the 349 cc K7 and the 498 cc K10. Jimmy Simpson rode a 350 to 3rd place in the Junior TT and won races in Europe, but in 1928 AJS used the overhead-valve engine in the TT. In 1929 there were again two machines with an overhead cam, this time the 349 cc M7 and the 498 cc M10. Wal Handley came 2nd in the 1929 Junior TT for AJS. The following year Jimmie Guthrie won the 1930 Lightweight TT on a 250 cc AJS.
In 1931, the AJS S3 V-twin was released, a 496 cc transverse V-twin tourer with shaft primary drive and alloy cylinder heads. It had been expensive to develop and was slow to sell. Even though it held 117 world records, the AJS company was now in financial trouble.