Summary

Spring loaded pressure plate for uniform feed regardless of filament thickness

See it in action during a fast retraction http://youtu.be/HMMkCDL2KWM

Instructions

What you need:
1 spring (11mm diameter)
1 M5 bolt longher than 7cm
2 printable caps

Instructions:

cut the head of the bolt to obtain a 6cm long thread
cut the spring and keep a 3,5com long piece
print the two caps
untwist the original retainer and substitute the threaded screw with the new one (longher)
thread the lower cap, the spring and the upper cap
put back the original retainer

enjoy