Minimalistic Mk8 Replacements

There's a few of these designs around, and while maybe veterans may already know, the purpose wasn't obvious to me at first so here's my attempt to explain the parts, function and reasons for this thing:-

* Firstly, the references to MK7 and MK8 are two versions of the same part - i.e. the top section of the replicator extrusion equipment.

* The stock Makerbot Replicator has a stepper motor that controls the feedrate of filament into the heater element below it.

* Pushing filament up against this stepper motor is a plastic "delrin plunger" - http://store.makerbot.com/mk7-delrin-plunger.html

* This plunger encounters wear due to friction, causing less pressure on the filament against the stepper drive. Eventually the filament will exhibit feedrate problems, causing symptoms such as premature melting down in the heater/nozzle areas below.

* Another cause of problems is that the plunger does not tolerate changes in filament diameter very well, if at all - such changes can