These are the self locking copper exhaust nuts to hold on the heat exchangers securely without backing off. very nice built in washer for a secure mating surface. special copper nuts are sold each. 12mm head 8mm hole.
Known as: lock ring a 6 / lock ring 6 / lock ring b 6 / lock ring a6 / lock ring m 6
Known as: lock ring a 6 brake 60 mm / lock ring a 6 din 127 phr / lock ring a 6.4