The root of that lag bolt should be about a half inch. Pilot it to about 5 to 5-1/2 inches with a 7/16 or 1/2 inch bit, then counter bore for the length of the shoulder with an 11/16; then drive the bolt home with a BFH, and give it a twist or two with a wrench to seat the threads. This from an old "Clumsum" who has driven far too many lags into old dead trees (power poles).