The extruder can be customised for particular pulley and idler sizes, spring length, etc, the up to date source is also available [here](https://gitlab.com/parts/extrudery/tree/master). The provided sample customisations (corresponding to the uploaded STLs) are for my two extruders. * E0 pulley diameter 12.65mm with the teeth 7.9mm from the bottom of the pulley * E1 pulley diameter 11.5mm with teeth 7.5mm from the bottom of the pulley both extruders use spring 10mm in diameter 9.6mm long when fully compressed. Both are 1.75 filament. Though there are many parameters adjustable, it is not guaranteed, of course, that any combination of parameters will produce a functional part, but at least that's a start. I use [these mounts](https://www.youmagine.com/designs/frame-mount-for-geared-stepper) to mount them on my once BQ Prusa i3 Hephestos printer's frame.