PURPOSE: To make the heat transfer efficient by arraying fins in rows having undulations with spacings between adjacent holes; the spacings of one row differing from those of the other.

CONSTITUTION: The assembly 10 has fins 26 composed of generally flat plates 27 having undulations 30 which are arranged in rows 32, 34, 36 between adjacent holes 28 and distant with spacings along the rows 32, 34, 36 such that one of spacings between the adjacent undulations differs from the next one. While a hot fluid flows over the fins 26, their undulations 30 always block the forming of a boundary layer and cause a turbulent flow of the hot fluid to enhance the heat transfer and the tops 40 of the undulations 30 are utilized as a surface area enhancing the heat transfer.