method of forming a cylinder head referring now to fig4 of the drawings there is shown a monoblock cast cylinder head 10 which is in accordance with the present invention the cylinder head 10 is illustrated to have a combustion chamber 12 and an exhaust port passage 14 the exhaust port passage 14 is shown to receive therein a core structure a valve seat 16 for an exhaust valve not shown and an improved curved port liner 32 are in their proper place as shown the inlet portion of the port liner 32 is supported by an exposed portion 33 of the port liner 32 which fuses together with the cylinder head during casting a space 20 also shown filled with moulding sand is formed between the outer surface of the port liner 32 and the inner wall of the body of the cylinder head 10 defining the exhaust port passage 14 and serves as a heat insulating layer in this invention the formation of the exhaust port passage 14 is made with an improved core structure 34 which is well shown in fig5 to 7 the improved core structure 34 comprises an improved port liner 32 which is curved and is made of a heat resisting steel place and is dividable into two semi cylindrical segments 32a and 32b as well illustrated in fig6 preferably the port liner 32 is dividable along an imaginary plane including in it the longitudinal axis thereof each of the segments 32a and 32b is provided at the base edges thereof with two pairs of projections or lugs 36a 36b the lugs 36a.