An 81 CB750 was not a sohc bike, this post will get moved to the 'other bikes' section no doubt.
I'm uncertain if the stock rear wheel for an 81 cb750K was a 17" or an 18", you've probably got a harley wheel laced up with aftermarket spokes to your bike's stock rear hub. It was a popular modification and still has it's appeal to some. ServiceHonda calls out a 17"
and an 18" rear tire for drum braked spoked wheel models(indicating an 17 or 18 wheel) but
BikeBandit only calls out an 18" rear tire for drum braked spoked wheel models.
The SOHC CB750 models all used 40 spoke wheels, I'm not sure about the DOHC models. That info may be available from the fine members at one of the DOHC specific sites like
www.CB1100f.net or
www.cb750c.comGood Luck.
Alan F.