Not sure if this was covered on here or not, but I seem to remember some folks were trying to find hexes to allow the use of 12mm hex wheels. I'm in the process of building an XB4 and I would really like to be able to do this too. I have heaps of Caster wheels which I really need to be able to use.
Came across these on HK.
http://www.hobbyking.com/hobbyking/s...12mm_Hex_.html
These are available in Blue, Purple, Red and Titanium and in 4, 5, 6 & 7mm thicknesses. It is a 5mm hole and they would appear to be identical 12mm versions of Xray's 14mm hexes. And they are cheap!
Only downside is that they aren't in stock in the UK, so delivery could be slow.
Jimmy