Don't mean to beat a dead horse on this topic, but I would disagree with Mr. Wolkner's explanation. While applying a given normal force over a smaller contact area would increase the interface unit pressure, it would have no real effect on axial separation of the mating faces, nor on the ability of that interface to transfer shear forces.
I concur Terry, separation between the mating surfaces is of course prevented by the preloaded Force
, not Contact pressure
But Herrn Wolkner works in sales so give him a break. (Sorry Don)
I still believe that the groove is there to ensure contact on both sides of the bolt, so that no convex contact phenomenon appears at high bolt-preoads, a metod widely used since the dawn of mechanical engineering when you think about it.
Actually, when this is a high-precision component, I would probably use some FEA to calculate a slight angle to be ground
on the contact-surfaces, all in order to assure perfect mating after a given preload.