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Ok, the M8_extruder_Block part is wrong. There supposed to be a hole where the big gear (with the M8 bolt) should go through.
See this picture:http://objects.reprap.org/mediawiki/images/thumb/2/28/M8_2inside.jpg/500px-M8_2inside.jpghttp://objects.reprap.org/medi...
For now, the hole which are nearer to the gear is a blind-hole.
Wade: why are you not creating this design in openscad?;-)
Maybe others could help to you.