Above : Square Four 1951 Right : Square Four redesigned engine 1954
The Square Four lasted , in varying shapes and sizes , for 27-years from 1931 until 1958 , surviving the Great depression and the Second World War . Following the first year of production the engine was enlarged to 597cc , which was particularly useful for sidecar owners .
OVERHEATING CYLINDERS The overhead cam in the original 497cc motor was very noisy and was later redesigned with conventional pushrods and rocker arms . Because the four-cylinder engine had lighter flywheels than a single , its acceleration was considerably faster ,
which gave the bike a sporty feel . It also had the ability to pull strongly from lower rpm . Attempts to increase power output merely increased the serious heating problems with the rear cylinders . Following Turner ’ s move to Triumph in 1937 , a new designer , Val Page , produced 4F and 4G models with revised 597cc and 997cc engines . The consistent overheating problems with the rear cylinders remained despite the new engines featuring more finning on the cylinder head , also a tunnel between the cylinders . At this stage the 4G was producing 34bhp .
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