1/4" by 28. I like the Harry Higley style. ( http://www2.towerhobbies.com/cgi-bin...?&I=LXL745&P=0 ) Great Planes also has them ( http://www2.towerhobbies.com/cgi-bin...?&I=LXK241&P=0 ) and they're a wee bit cheaper.

Luxury Yacht (AKA: the aviator formerly known as Dave)