This is a replacement E3D V6 heater block that is used in the E3D V6 hot end for both the 1.75mm version and the 3mm version.

The heater block is basically a small machined aluminum block. It has a CNC drilled and tapped thread right the way through the top that accepts the V6 heat break and E3D nozzle. It also has CNC drilled holes that accept the heater cartridge as well as 100kΩ 1% NTC thermistor. The heater cartridge is used to heat up the heater block to the desired temperature which intern heats up the hot end of the heat break and nozzle allowing smooth extrusion of the filament. The Thermistor is used to read and monitor the temperature of the heater block allowing for accurate closed loop PID temperature control.