[suicideneil]

Blue loctite on the shaft and grubscrew ftw- never had an issue myself using that technique ( allow to dry overnight is important too yeah ).

One other option is to drill or grind a little dimple onto the shaft in the right place ( on the flat spot ), then re-attach the pinion and tighten the grubscrew down into that dimple- that'll stop the pinion wondering along the shaft ( use blue threadloc too ofcourse ).