This is an upgrade I invented just from boreness. This spacer allows You to adjust the spring force on single drive extruder. Great for long bowdens or worn out drive gear.

All You need is to unscrew one of screws that holds the spring in place and screw it from the other side. Now place the spacer on a screw (should fit quite tight and snap on it) and put everything togerher. Now by turning the screw, filament clamping force is adjusted.