Rotax Clutch Drum Fixation Nut. This nut holds the clutch pack, drum, and associated washers in place during operation. Made of hardened steel, this nut is a specialty milled to fit onto the end of the crankshaft on Rotax Max Evo engines.
Each nut is M10 in size, by 1.0 thread pitch. Typically, one needs a 17mm impact with a medium torque setting / speed to install and remove this nut.