History of the Square Four (1931-58)

Ariel ran into financial trouble in the late 1920's and closed shop for a short period while the founder's son, Jack Sangster, took over and restructured the company. With his life savings he bought all of the tools & re-hired the best of Ariel's staff, then moved 500 yards down the road to a new factory. Then in 1928 Edward Turner proposed a very interesting new engine design, sketched on the back of a pack of Wild Woodbine cigarettes. He had shown it to the motorcycle factories to find a buyer for the innovative design, but only Sangster at Ariel was interested. He offered Turner (below with 1931 4F) space at the new Ariel factory as well as the services of a junior draftsman, Bert Hopwood. After 18 months of intense development (mid-1930) the Ariel Square Four was unveiled internally.
Four cylinder engines had been built in a variety of configurations for 25 years previously, and included the in-line four, the v-four, and the transverse four. Turner decided they were inefficient and too large. He proposed the Ariel Square Four, the only motorcycle to have four cylinders arranged vertically and equidistant to form a square. The front two cylinders drove one crankshaft, and the rear two drove another. A helically toothed flywheel gear was inbetween. When these gears meshed, the crankpins of one shaft were at top AND bottom dead centers while the crankpins of the other were at half- stroke. This resulted in perfect balance and no vibration at any speed, all in a very compact unit.
At its public unveiling at the Olympia Motorcycle Show in late-1930 the 1931 Ariel Square-Four (above left) caused a huge sensation, even over the new Matchless Silver Hawk narrow-angle v-four OHC. The first Square Four production models were 497cc lightweight touring bikes but the public wanted more power so Ariel increased the capacity to 597cc in 1932 models (above right 3). Since the four air-cooled cylinders were set in a square formation (two 180ยบ parallel twins placed back to back) and the OHV gear was operated by a chain-driven camshaft, the early Square Fours experienced inadequate cooling & therefore overheating. But in 1931 Ariel won the much coveted Maudes Trophy, a test of durability, reliability, and ease of use. The famous Ariel 7 test involved seven different Ariel models attempting tests related to the number 7. For the Square Four, seven schoolboys kick-started it 7 times each (above right). The Square Four started on 48 of 49 kicks. It also averaged 60mph for 700 miles!
In 1933 the speed potential of the Square Four was obvious but unproven until Ben Bickell rode a highly-modified 600 with supercharger at the Brooklands track, lapping at over 110 mph and only narrowly missing becoming the first British 500cc to achieve 100 miles in an hour.
The Model 4F Square Fours include the 500cc and 600cc engine layouts. They have overhead cam (OHC) engines although a small number of 4F's were manufactured in 1937 with 600cc overhead valve (OHV) engines. The OHC models are commonly referred to as "cammers" because of their camshaft and cylinder layout.
1935 cammer 4F 600cc 1936 cammer 4F 600cc 1937 4G 1000cc 1938 4G 1000cc
In November 1935 the Square Four went through a complete redesign, which replaced the camshafts with pushrods and the crank and crankcase were completely changed, now producing 997cc. The new 4G reduced the 4F "cammy's" tendency to overheat around the cylinder head. To prove the new design & gain publicity, Edward Turner had Freddie Clarke ride a 1936 Square Four 1000cc to Brooklands track and ran "ten mph to a hundred in top" (above far right). By 1937 the British rearmament program put people back to work and motorcycle sales skyrocketed and the Square Four became "monarch of the multi's".
1948 Mk I Mk I engine 1949 Mk I 1952 Mk I
In 1949 the Mk I 1000cc was introduced (above), and the engine became an all alloy type (reduced weight by 33 lbs) with the overall handling and acceleration improved. This style was built through 1953. The popular Mk II 1000cc (below) was built from 1953-58 and featured the 4-pipe exhaust system. Its revised cylinder & head layout featured separate cylinders vs. the Mk I monobloc casting to promote more efficient cooling.
1955 Mk II 1955 Mk II 1957 Mk II
Over its 27 years of production, the Square Four evolved through progressive advancements: a chain tensioner, swinging arm rockers vs pushrods, and a special ducting system to transfer cooling air to the rear cylinders. It was so well-known for its reliability that law enforcement agencies used it as their vehicle of choice.
Australian Police Canadian Police

Square Four Production

Year Range











Mk I