frusto conical in shape and sized to mate with the corresponding ports 47 the interior of the port inserts 49 is in the form of a venturi to enhance the flow of gases from the sole flue 17 or double sole flue 25 into the regenerator chamber 15 or double regenerator chamber 27 as the case may be it should be noted that due to the long length of the sole flue 17 or double sole flue 25 there is a pressure drop as gases flow therethrough thus the size of the venturi form of the interior of the port inserts may be varied along the length of the sole flue 17 or double sole flue 25 to compensate for this pressure drop the object being to equalize the volume of gas per unit of time passing through all of the ports 47 thus producing a uniformity of gas flow through all portions of the regenerator chambers 15 or double regenerator chambers 27 respectively referring to fig3 it will be noted that there is a pair of notches 51 formed into the otherwise rectangular face of each of the divider liners 29 and side liners 31 these notches 51 are positioned at two consecutive corners of the liners 29 31 and are in the form of a 90 u00b0 angle with a radiused corner the ends of the liners 29 31 between the notches 51 is referred to as the notch end 53 referring to fig5 as well as fig2 it will be noted that.