Origin
The earliest IOE layouts used atmospheric inlet valves which were held closed with a weak spring and were opened by the suction created when the piston went down on the inlet stroke. When the limits of this system were reached, the design was improved without substantial changes to the head casting by adding a mechanical system to open the inlet valves and stronger springs to close them. In both cases, the exhaust valves were in the block and were opened by contact with a camshaft through a tappet or valve lifter and closed by springs.
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