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In 1920, AJS production models were all equipped with side-valve engines but the company entered a new overhead-valve 350cc model in the Isle of Man Junior TT that year and its rider, Cyril Williams, had such a convincing lead at the end that he was able to push it nearly four miles, albeit mostly downhill, to the finishing line to win the race.

The following year, Tom Sheard rode one to victory in the Junior TT and Howard Davies won the Senior TT with another – the first time that a 350cc machine won both the Junior and Senior TTs.

The H6 production version was launched in November, 1922, and soon acquired the ‘Big Port’ nickname because of its large exhaust port and pipe. Its right-first-time, racer-on-the-road design meant that it was ridden to achieve countless sporting successes in the hands of privateers and it underwent only minor detail changes until it was superseded by the much-revised M6 model in 1929.

For many years, this particular example was in the collection of the late Jim Noble of North Leeds, UK. It is to original specification with acetylene lighting, a generator, a Cowey speedometer, a Lagrande leather saddle, a luggage carrier and two panniers. It was acquired for the NZ Classic Motorcycles collection via a UK auction house from a private vendor in Ripon, Yorkshire, in July, 2009.

The 1922 machine was a classic design that would become famous as the ‘Big Port’ on account 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 and in production form (first offered to the public in 1923), was also AJS’s most popular sports motorcycle throughout the 1920s. At this time, the company produced a comprehensive range of other models ranging from 250 to 1,000 cc. These were generally given a model number, plus letter to denote the year of manufacture (for example, B meant 1924, F 1925, G 1926).

In 1929 for example, the AJS range consisted of: M1 Deluxe 996 cc side-valve V-twin £76/10/0; M2 Standard 996 cc side-valve V-twin £66/0/0; M3 Deluxe Touring 349 cc side-valve single £48/10/0; M4 Deluxe Sporting 349 cc side-valve single £48/10/0; M5 Standard Sporting 349 cc side-valve single £45/0/0; M6 349 cc overhead-valve single £54/10/0 (twin port), £52/0/0 (single port); MR6 Special Sports 349 cc overhead-valve single £62/0/0; M7 349 cc overhead-camshaft single £62/0/0; M8 498 cc overhead-valve single £62/0/0 (twin port), £59/10/0 (single port); MR8 Special Sports 498 cc overhead-valve single £72/0/0; M9 Deluxe Touring 498 cc side-valve £54/0/0; M10 498 cc overhead-camshaft single £72/0/0; M12 Lightweight 248 cc side-valve single £39/17/6. Several of these were intended to pull one of the 12 AJS sidecars also on offer, including sports, touring and commercial models.

By 1927, it had become clear that push-rod overhead-valve designs were becoming dated in racing, 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 third 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 second in the 1929 Junior TT for AJS. The following year Jimmie Guthrie won the 1930 Lightweight TT on a 250 cc AJS. The R7,350 ohc, won 8 of the 9 Grand Prix and established at Montlhery, near Paris, a number of world records, including 1 hour at an average of 104.5 miles per hour and 2 hours at an average of 99.5

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.