Abstract: The conversion of a multi-cylinder liquid cooled internal combustion engine into a powered air compressor by placing inserts into parts of the cylinder head above certain of the cylinders after removal of the valve gear therefrom. The insert seals the coolant jacket in the cylinder head and provides surfaces for receiving a proprietary air compressor valve. The insert may be arranged to connect with ports in the cylinder head in which case the engine manifolding must be modified to separate the compressor air flows from the engine gases, although where the engine receives air as an inlet gas (e.g. a diesel engine) the inlet manifold can be used to provide air to the converted cylinders as well. The cylinders selected for conversion should be chosen having regard to the balance and smoothness of running of the remaining cylinders used to drive them.