What the standard front and rear sprocket size for a 1974 CB750K?
18T front, 48T rear.
If you're not touring a lot, try a 17T front for a snappier ride and better MPG: the 18T is overgeared. 
Didn't they do that 18T thing to help reduce chain breakage/ engine case breakage?
Yep, that was their official statement on the topic.
The story behind that story, though, was more like, "Americans are really flogging these bikes and we are suffering large warranty costs for replacing engine cases. If we increase the gearing to the point where the bike won't do burnouts in 1st gear we can discourage that sort of activity."
I don't know the Japanese translation for that, though.
The K0 started with 16T front and 45T rear, which earned the bike the nickname "The $1495.00 Chainbreaker" (original price was pretty low). Before 1969 was over, the kits were appearing to change it to 17T front and 48T rear to reduce the severe wear on the final drive. The kits came with a (silly) sheetmetal guide that was supposed to encourage a broken chain to go around the front sprocket and eject out the back, but it usually just gnarled up into a ball when the chain went too slack (from neglect) and then BROKE the chain, instead. So then the endless chain came out (lots of fun to install). By late 1971, on the K2 model, they simply installed the 18T front sprocket with no other changes. During the "new factory" series of the later K1 they had slightly opened up the cases in the sprocket area, so it could fit this new, big sprocket. But, it became so overgeared then that the top speed dropped from the typical 132 MPH to about 126 MPH. We frequently changed to the 17T when customers complained of lackluster performance, and most of them never knew what it was we did that "really fixed my 750!".
