The constant demand by engine builders for increased brake mean effective pressure (bmep) has made piston and ring temperature a serious limiting feature of engine development. The various methods previously employed to cool the piston are contrasted with the new approach of coring the oil-cooling passages by means of a water-soluble core. The development by Wellworthy of three practical foundry techniques embodying this principle which also permits the use of armored top grooves is described, with typical examples of such pistons. The effects of cooling in terms of piston temperature distribution are shown. The technique so far covers pistons 3 to 10 in. in diameter, and is fully patented.