This spacer makes it possible to adjust the height of the heater blocks individually if doing experiments with dual extrusion on the Ultimaker 2.
The original spring proved too weak in certain conditions and the solid spacer that replaced the spring on the UM2+ makes hight adjustment impossible.

NOTE: This design is experimental, you might need to scale the stationary part (insert) for the threads to get a tight fit. Too loose fit and the spacer might collapse under load.

Only one of the spacers needs to be adjustable.
The I2K version is a shorter version which is suitable if you are using the I2K washer from 3dSolex in between the PTFE and the heater block.