Well you lucked out catching me as it's just before Midnight here but I'm watching an old movie.... lol
Went and put a measuring device on mine and it looks to be a 1 1/4".
http://www3.towerhobbies.com/cgi-bin...?&I=LXFU88&P=M
I don't think it comes with wheel collars either???? Been too long since I put it together.