“5/8 mm”???
Ooops, 5/8" hex!
“5/8 mm”???
denso plugs fit and forgetNormal! Normal! you cant get anything more normal than a 14mm spark plug
The first time I even saw anything different was in the late sixties when Ron Kemp put a 10mm spark plug in the center of his Flash cylinder head to "light the bonfire in the middle" I did not think anything except 14 and 18 mm plugs existed (apart from model airplanes)
I believe other motorcycles from "over the water are available with smaller plugs...(even I have one now)
Its a bit like chains and tyres once upon a time it was easy with a limited choice now its just another maze to traverse!
1.25 pitchI agree with @MSVH Y3 that your plugs probably suit a 5/8" AF plug spanner. I had Citroen BX that had 14mm plugs deeply recessed in the head, which required a special very long box spanner (long tube with a hex shape pressed on the end and an integral tommy bar). I no longer have the car, or any plugs for it (tapered seat BTW), but I still have the spanner. It measures 5/8" AF. So plugs with that size hexagon exist with a 14mm thread.