<https://www.yesterdays.nl/product/ariel-1937-4g-square-four-1000-cc-4-cyl-ohv/#product>
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"Ariel 1937&amp;nbsp; model 4G&amp;nbsp; 997cc Square Four&amp;nbsp;&amp;nbsp; frame # P576&amp;nbsp; engine# DC565    Ariel unveiled the all new&amp;nbsp; SQ 4 to the motorcycle world at the Olympia show in 1930. It was a brainchild of design genius Edward Turner and was powered by an OHC engine with 2 parallel crankshafts, mounted in a crankcase transversely across the frame.&amp;nbsp;  The two cranks were geared together in the center uf the unit, so in effect there were two vertical twin engines running in opposite directions.&amp;nbsp;  The result was a silky, smooth power output with instant response to the throttle.&amp;nbsp;  In 1932 the capacity was increased to 600 cc&amp;nbsp; and at the Olympia show&amp;nbsp; in&amp;nbsp; November 1935&amp;nbsp; a new&amp;nbsp; 4&amp;nbsp; was unveiled , this time of 1000 cc and with pushrods instead of the chain-driven overhead cam.&amp;nbsp;  The roller&amp;nbsp; conrod bearings&amp;nbsp; were also dropped in favour of&amp;nbsp; plain bearings and the coupling gears were moved from the center of the engine and placed in a case on the left side. Power output was about 36 HP, top rpm approx. 5800.&amp;nbsp;&amp;nbsp;  This new model attracted much attention but the public had to be patient because the first machines were actually produced almost a year later, in September 1936.&amp;nbsp;  This well-restored&amp;nbsp; early&amp;nbsp; &amp;ldquo;Mark 1&amp;rdquo;&amp;nbsp; is a sought after model because of its smooth acceleration ( 10-100 mph in top gear!), reliability and compactness."
<https://www.yesterdays.nl/product/ariel-1937-4g-square-four-1000-cc-4-cyl-ohv/>