A563-C

for use with a325 plain finish bolts.
a six-sided internally threaded fastener which is both thicker and wider across the flats than a same-sized finished hex nut.
this is the strongest of all comparably graded nuts because of its greater length of thread engagement and greater resistance to dilation (widening or stretching).
grade c hex nuts have a rockwell hardness ranging from b78-c38.
grade c hex nuts offer a proof load of 144,000psi.
fully traceable certifications of chemical and physical compliance available by request.