Early studs are m8, later (72 on?) are m10
PS the partzilla fiche has it wrong, showing the later m10 stud used in 70.....
conveniently the part # shows the diameter
Yamaha OEM Exhaust Stud Bolt 90116-08166-00 the 08 is the diameter
BOLT, STUD 90116-10195-00 the 10 is the diameter
But why not just use a bolt? spin on a nut, thread bolt in, with the strongest grade locktight, use the convenient hex head to tighten, then cut to length after you installed it. Unthread the nut to reshape any damage done while cutting it off. Done.