On 11/25/2012 7:50 PM, greg at gelhar.com wrote: > Looks like you ar installing a 5/16-24 Helicoil. Use the 21/64 drill. > Trust the engineers that designed them. ...with a brief moment of consideration as to whether one really wants a fine thread in aluminum in the first place, and whether it wouldn't be possible to use 5/16-18 instead. John.