It's a common 5/16" stud, coarse thread on the head side, fine thread for the nut. I replaced mine with ARP stainless studs (1.625" over all length) and 5/16"F all metal self locking jet nuts which really helps with socket access as they have a reduced hex head. No need for the nut retainer plate when using the jet nuts, which come from the aircraft industry, btw. In the U.S. jet nuts are available from Aircraft Spruce and racing suppliers like Pegasus Racing among others.
Photo shows my ARP install before I changed pipe ends.
