HST double bolt bowden extruder V4

this is an advanced version of "HST double bolt bowden extruder" (http://www.thingiverse.com/thing:437949)
now with access to clean bolts and one central screw for tensioning. This bowden extruder is quite big, but it transports the filament very well without any problems during a long period of time.
Parts:
2x Hobbed Bolt: diameter = 8mm, length = 24 mm
2x Bearing 608zz
4x Bearing 688
2x M8 Hexaon Head Screw length = 50 mm
2x M8 Nut
4x Washer (diam-int = 8,4 mm / diam-e