A ported cylinder for an opposed-piston engine has a bore to support movements of a pair of opposed pistons and longitudinally-spaced intake and exhaust ports that are controlled by movements of oppositely-disposed pistons in the bore. The bore includes a central portion where combustion occurs and respective end portions. Each end portion extends from an inner edge of a port to the nearest open end of the cylinder. In the central portion the bore has a standard diameter throughout. The bore has a second, larger diameter in each end portion. When the cylinder liner is cold, the bore diameter transitions continuously in size, from the standard diameter to the larger diameter, in an area running from the central portion to an inner edge of a port. From the inner edge, the bore is of the larger diameter throughout the end portion.