This is a spacer for the MK4/MK3 extruder. It prevents the bolts on the bottom of the z-stage from colliding with the bolts on the build platform.Back the filament out of the heater module, then remove the extruder from the Makerbot. Unscrew the bolts holding the heater module to the main body of the extruder, and remove them. Place the spacer on top of the insulator retainer. Replace the bolts that you removed with 16mm ones, and reinstall the heater module.