[grbl Issue#1609] grbl for lathe z axis infinite movement

hi all
I’m finishing a metal cnc lathe, I used gbrl with arduino as a controller slave from a rpi with wifi, it have x, y movement for the tool and z axis spin the jaw
When configure the z axis gbrl ask for a max movement of lathe if I put 0 lathe don’t spin,
if I insert a number lathe get finish his movement at input number
What need to change in grbl code for infinite movement in z axis in 2 directions?