Good point about plug length and it can be confusing. Both OS and Enya plugs are, for some unknown reason, what must be called a medium length plug. Ideally a plug should be long enough to come flush with the surface of the combustion chamber. Strangely, OS plugs are too short for OS engines as can be seen in the photo of an OS 40 cylinder head with a normal long reach on the left and an OS plug on the right. Enya heads have a better fit with the medium length plugs though. Just about all engines, other than maybe some very small ones, are designed to take a true long reach plug. So although an OS #8 is usually called the "standard" it'll actually be too short even if it's a suitable heat range.