Resetting A-axis

I did some manual G-code to use a 4th axis rotary basically like a lathe.  The A axis doesn't have a homing switch.  It advances a specified number of turns as the X and Y does its thing.  

Part gets changed out, repeat.  

What's the correct G-code to reset the A axis?  Numerically, it's spooled up like 1000 turns in one direction.  Moves aren't relative and I don't need the thing to unspool 1000 turns to get back to the absolute A-axis value the next G-code run will start at.

 

I tried G10 L20 P0 A0

This doesn't work. It's resetting the work coords to 0 so the G-code works at first, but continues incrementing A0 indefinitely in machine coords. After about 10 runs, the G-code faulted mid-run, says the next command would move joint 3 past machine limits.

At that point, A has reached 9800 or so, ini limited A to 10,000. I can put in a larger number. but we're just kicking this can down the road.

Can I reset the machine coord value, or is there a way to say the A axis has no limits on how many times it can turn?