The optimum expansion is the one that develops a tight joint with adequate strength and with minimum stress. The Torque needed for achieving optimum tube expansion varies with the tube diameter, tube thickness, tube sheet thickness, tube material and tube sheet material. Obviously we must determine some relation between the amount of tube expansion and the amount of torque required to achieve that. Many feel that the manufacturer should specify the torque needed for particular combination of various 
conditions. Since many preferential factors come into play like the desired amount of expansion, the lubricant used, the expansion length set on expander etc. it has to remain with the user to arrive at the torque figure.It is recommended that a group of about five tubes be expanded, measuring the results after every expansion. Comparison of the measurements with the calculated figures will enable decision making on the increase or decrease of the torque to be applied. Once the correct torque is determined the same has to be repeatedly applied for all tubes.

One of the widest method of determining tube expansion is to determine the percentage of wall reduction of the tube being expanded. The wall reduction is due to thinning of the tube wall after the tube outer diameter comes into contact with the tube sheet bore during Tube Expansion. Tube walls of non-ferrous tubes in condensers are reduced by 3 to 4 % to get an optimum joint. A wall reduction of 8 to 10% in the case of ferrous tubes in Heat Exchangers is considered optimum, whereas non-ferrous tubes require to be reduced by 8 to 12% due to the pressure involved.