I could go home and make this measurement for you, but you'd be incredibly lucky to find anything close, anywhere. The U-bots used in the rotors are specialty items, stainless steel, very long threads -- I've never found anything close, anywhere, except buying spares made by the rotor manufacturer themselves.
Luckily, those are readily available! http://www.rotorservice.com
, which is Norm's Rotor Service in Maryland, is one dealer who stocks these U-bolts and can ship them same-day.