This is designed for use in an MK7-style extruder in the case where a 6.5mm OD polyethylene tube is used to guide the filament to the extruder. This design has been successfully used with 100% reliability for many hours. The extruder pictured was originally a QU-BD model. I could not get the original extruder, as shipped, to extrude in any useable fashion. The modified extruder is now working reliably (the original QU-BD gear has been replaced with a Makerbot style extruder gear and the system for providing traction to the extruder gear has been replaced with the very-well-thought-out Minimalistic Mk7 Extruder parts.