Plain Plug Gauge for Proximal Hole ID of the Nail. Gauging area should have lapped finish. (Go 6.600 to 6.603 Cyl. 0.001; Length 20 to 21 or Std & NOGO 6.648 to 6.650 Cyl. 0.001 10 to 11 or Std Gauge No. PPG 6600.6650