I remember seeing something mentioned here about using CB750 rings in the 13, I was looking thru some of my old paperwork on the 13, a lot of it was owned by a mate of mine who passed away from cancer, he bought a 79 model and kept it going for many many years, and he was kind enough to leave me the spare bike and a lot of bits and pieces, anyway in one of the manuals he does mention using CB750 rings, they are 61mm, and a one piece oil ring, so I guess next time I do a re-build i'll buy some of those, checking the prices they are a lot cheaper than 1300 rings even though you have to buy 2 x 4 cylinder kits, cheers all