UCCNC seems close to G76 in LinuxCNC http://linuxcnc.org/docs/html/gcode/g-c ... #gcode:g76
I- - The thread peak offset from the drive line. Negative I values are external threads, and positive I values are internal threads. Generally the material has been turned to this size before the G76 cycle.
Uccnc Manual:
I: The thread peak position in units along the X-axis. This is the starting height of the thread and the material was mostly turned down to this size.