5/16 X 32 thread. From Jim Chambers. Extra large, deep inside cone improves ignition. Unit is installed using a vice grip, then shoulder is cut off with a hacksaw.
The large cone of this touch hole liner brings the flash to with in a few thousandths of an inch of the main powder charge which will noticibly decrease the ignition time. Use letter drill "I" and 5/16 x 32 tap.