Generally, the extruder for a RepRap 3D printer is referring to both the device that drives the filament (the cold end) and the hot end that melts it. In this recipe, we'll take a look at some issues that arise from the cold end of the extruder.
Extruder designs have become so prolific that a book could be written about this mechanism alone. However, most of them are quite simple and work with a common principle; a filament is sandwiched between an idler bearing and a toothed gear or bolt (called the drive gear from here on) that's motor driven.
Familiarize yourself with your specific extruder system. Yours may be attached directly to the hot end, or it may be separated by a Bowden Cable system. In either case, the following recipe will be useful.
Be prepared to make some adjustments that will require basic tools. Precision digital calipers or a micrometer would be ideal for measuring.