Freeze Plug Question

Unless you have coolant / water in the camshaft cavity, brass cup plugs would have no advantage over steel. Brass cup plugs are offered for use where coolant / water is in contact with the cup plug as brass has superior corrosion resistance when compared to steel.
Also the water jacket cup plugs are not ‘freeze plugs’. Those side water jacket openings allow the foundry to drain out the core sand required in the casting process and also to position the sand cores in the casting mold.