I believe it's 1/4x16 acme threaded rod, or whatever came on the printrbot simple metal. It's stock factory as far as I know, I bought the printer used and it was never replaced that I know of. Any way i dropped the new Marlin in it today (2.0). Now it homes to the 0 position, but if I put an offset in the firmware it still refuses to lower the nozzle any lower. The lcd and slicer both register the offset, meaning if I put the offset in Marlin firmware at -1.00 or just 1.00 and home the Z, after homing is complete it will now show whatever value I set as the offset. However the nozzle is not higher or lower. I'm using paper as a feeler gauge and the nozzle height is always the same, doesn't matter what it says the offset is. I don't know.