It’s not possible that the rocker
arms directly fixed to the valve tops due to metal expansion as the engine
heats up during running period. If the rocker arms didn't have a clearance
between them and the valve tops, expanding metal would make the valves be
always fractionally open.
So a very small clearance for heat expansion is
necessary, which can be adjusted by means of a screw and locking nut.
This valve or tappet clearance is
normally five thou for each cylinder's of two inlet valves and seven thou for
each cylinder's of exhaust valve. The clearances are different because the
inlet valves, always bathed in a rush of cool fuel and air ,so less possibility
to expanding than Exhaust valve; where exhaust valve constantly bombarded with
hot and burnt fuel mixture. For Waukesha VHP5904 GSID adjust the clearance
after every 4000 Hours.
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