I'm guessing the OP is probably asking the same questions I found myself asking - what's the point of/reason behind the special nut that appears to be used on the underside of the C/CA/CB/CL77 crankcase, where exactly does it fit and why's there only one of them?
If you look at the left hand side of the engine with the cover off, you'll see an opening for an oil galley on the bottom edge. This is an image of the left side bottom case half (upside down); the galley opening I'm talking about is the elongated D shape:
Directly above it is a hole for one of the large case studs and to the left of that (in this image) is a hole for one of the smaller case studs. The nut - together with a sealing (copper) washer - goes on that stud. The reason it's there is to prevent leaks from the oil galley, and there's only one because there's only one stud that goes directly through an oil galley.
As anyone familiar with those engines knows, they were over-engineered because they were made at a time when Honda was getting to grips with moving to new markets (America in particular) and refining its industrial manufacturing processes. Rather than take chances with oil leaks, Honda took a belt-and-braces approach with the sealing washer and cap nut. And if you look at later bikes, it subsequently redesigned the engine cases in such a way that they didn't have any studs penetrating lower cases, eliminating the need for special parts.
Hope that helps - and welcome.
