...2mm...AON ELECTRIC has a motor called a 5800L (i have one if you want it) that is a 400 size but cant be fitted into a 540 position because of its funky adaptor face...It uses a 3.2mm shaft which will open up your choices a little bit...I was running it with a Mamba 25 in my Dark Impact and it was ok...A little warm but no big problems...Just another option maybe?