SummaryReplaces the rather crappy original (a screw and a washer - at least that's how mine came) There are 2 versions - the Mk6 has extra teeth on the bottom for extra grip ! I also put a drop or 2 of Supaglue on to stop any slippage - the original design was, IMHO, very prone to stretch then break at the 3mm attachment hole I usually print at least 2 - makes the top of the print a lot firmer - hardly any filament ooze with 2 It may also be useful on other 3D printers with a similar belt system.