an annular recess formed in the cylinder head at the combustion chamber end of the port 18 and comprising a cylindrical outer wall 34 adjoining a planar end wall 35 the outer wall 34 includes an inwardly protruding portion 36 that extends under and in engagement with the chamfer 32 at the outer edge of the seat insert to positively retain the insert 20 within the recess in the installed condition as shown in fig2 the center portion 33 of the insert 39 s seat end 28 and an annular surrounding portion 38 of the combustion chamber wall are machined at a shallow angle to form a smooth sided outwardly dished portion surrounding the valve seat 31 which improves the flow of gas entering or leaving the cylinder through the port 18 when the valve 22 is opened for reference purposes it is noted that the valve seat 31 formed on the insert 20 is centered on an axis 40 when the insert 20 is installed the axis 40 is also the reciprocating axis of the poppet valve 22 and the axis on which the cylindrical outer wall 34 of the valve seat recess is also centered in reference to any hypothetical plane normal to the axis 40 the chamfer 32 preferably forms an angle of about 45 u00b0 longitudinally away and.