4 or 5 mm bolt hole.
Radix blades fly light and quick for agressive 3D but they still do forward flight like they are on rails.
In addition to their smooth precise flight characteristics they are also one of the quietest blades in the air.
To top it off they auto great. The perfect combination for a 3D blade, ideal for the Trex 700, Synergy or other 90 size performance helis.