Depends on the inserts.

5/16" if you use the standard inserts. I have a small grinder and I can reduce the insert to the diameter of the shaft, 9/32". You can also get the Easton inserts, which fit nicely.

 

5/16" if you use the standard inserts. I have a small grinder and I can reduce the insert to the diameter of the shaft, 9/32". You can also get the Easton inserts, which fit nicely.

I use 9/32 on my Axis and FMJ shafts. My Victory shafts are larger diameter than that. I use the 5/16.

 

v force is 5/16