KD Tools 715 Universal 7/16" to 3/4" Nut Cracker
This universal nut cracker is used to split most stubborn nuts 7/16" (11mm) through 3/4" (19mm) across the flats.  The parallel action of the pusher cracks nuts without damaging bolt threads.  It's simple to use, just tighten the screw until the nuts splits.  Also this is a serviceable product.