Most people say 50 to 55 KE. But thats for a regular size head ( 1 1/8" to 1 3/16" ) Some of the great advantage of mechanicals is the large cutting diameter. (example My Wasp Jackhammer 100 have a cutting diameter of 1 3/4" ). If you shoot larger cut mechanicals I' d strongly recomend KE over 60 to 63. Good luck and if you' re not sure or on the edge.......shoot fixed!!!