Peter Schmehl and I designed a modded drive block that supports filament completely - all the way to the barrel / thermal barrier. If you have any filament giving you trouble at the point between the drive gear and the barrel, curling and jamming the extruder, you will find this design quite useful.

These models were printed on a MakerBot Thing-O-Matic and adjusted for Thing-O-Matic tolerances. Both have been tested in ABS and PLA. Depending on your calibration, it is likely that some handy work will be needed to get the fit of the plunger just right. Don't worry - very little effort is involved in prepping these parts for your extruder.

1) Print the two .STL files -- the front and the back of the drive block.

2) Disassemble your MakerBot MK7 drive block parts and set aside the two M3 screws which secure the old drive block to the motor. Do not disassemble the whole extruder! You only need to remove the fan, heat sink, old drive block and motor.

Be careful, also, to keep track of the plunger parts inside. These include a derlin plunger, two plastic washers, and 3 metal washers. These parts are tiny and easy to lose!

3) Before connecting the new parts to your extruder, some prep is involved. WITHOUT putting the plunger parts in to the drive block, use the two M3 screws to secure the front and back printed parts together. You don't need to mount them on the motor yet. Just secure them together.

4) Use a 5/64" or comparable (1.9-1.95 mm) drill bit to smooth out the filament holes. These are the small holes that run from the top to bottom of the drive block on either side of the drive gear. The goal is to make sure that there is little resistance on the filament as it runs through the block. Test out your smoothing by running a piece of filament through the holes. There should be little wiggle room, but the filament should slide easily.

5) Now you are ready to mount the parts to the motor. Unscrew the printed parts and assemble your extruder as usual makerbot.com/docs/thingomatic/instructions-for-educators/stepstruder-mk7-assembly/.

Take a look down the filament holes after everything is mounted to the motor to be sure that your plunger is centered on the drive gear. You may need to adjust the position of the drive gear on the motor.

6) Test the extruder by extruding some filament via the ReplicatorG control panel. If the plunger fit is too tight, use an xacto blade or a file to shave down the back of the plunger cavity just a bit.

Voila! Happy printing!