Why check bumping clearance in Main Air Compressor but not in Aux Engines

In air compressors requirement is to discharge the maximum amount of compressed volume of air out of 1st stage cylinder so that maximum cool fresh air should come in.
If the bumping clearance is more then air left inside stage will be more and at higher temperature after discharge so less mass of air will get inside in the suction stroke now due to this there will be increased suction temperature of air and consequently less work done and reduced efficiency of air compressor.
Where as in Aux Engine we need maximum air to remain in cylinder head at TDC for better and efficient combustion of the fuel . Thats why enough clearance is provided between piston top and underneath of cylinder head . No danger of contacting or banging of piston to cylinderhead.

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Manganese bronze is not typically used as a propeller material .Why

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