These John Guest 4mm Half Cartridges are what's used in the EZRstruder as well as our HE series hotends. They allow the 4mm PTFE tubing to pass thru giving a fully lined path for filament to travel reducing friction and chances for jamming. Sold individually, they press into an 8mm hole and have a plastic retainer with metal barbs that engage to hold the tubing. Search for our lanyard for PTC fittings to give you a positive locking connection on these fittings