Suck, squeeze, bang, blow. At the turn of the last century, if there was one man in America who really understood the workings of the internal combustion engine, it was Oscar Hedstrom.
The inventor and designer of Indian motorcycles built his prototype in 1901, only six years after Count Albert de Dion and Georges Bouton introduced the first ‘high speed’ four-stroke engine. Like the French original, the Hedstrom engine had an automatic inlet valve operated by cylinder suction, positioned directly above the side exhaust valve – a design that would become known as the F-Head.
The inlet valve was housed in a bronze casting that carried the mixture from the carburettor. This valve used a light spring but no rocker, cam or pushrod to open it. When the piston moved down the barrel the valve was simply sucked off its seat by the partial vacuum in the combustion chamber. As the piston moved back up the stroke to squeeze the mixture of air and petrol, the spring helped return the valve to the closed position, and cylinder compression pressure kept it there. The Big Bang caused by the spark plug would send the piston back down the cylinder and spin the flywheels with enough energy to propel the Indian motorcycle through the direct chain drive, and at the same send the piston back up the bore to blow the spent gases out of the exhaust.
Hedstrom’s engine had a
