Steel “G” fin fresh off the machine. Posted on July 15, 2013 by andy.worrall Fresh off the machine. This is a good example of our steel “G” fin on a steel tube. The fin is 12.7 mm high on a 25.4 mm O/D tube at 6.4 FPI (Fins Per Inch). Please check the web site for the other fin heights or FPI’s we can achieve with steel fin.