The correct (reduced head) 5/16" BSC thread nut, used to retain both top and bevel castings, 4 nuts are used per casting.
Our nuts are CNC manufactured to the original 'Reduced Hex' Norton dimensions but are in stainless steel, and the hex faces are milled - so they also look very pretty, which is important as they are a particularly prominent and visual part of the SOHC engine
As a background to these nuts - 'Reduced Hex' means that the Whitworth hex size (spanner size) is smaller than would normally be available for a 5/16" thread - and they did this because space is so limited around the bevel castings. If you own a Norton single already - you will know that this was common practice for a lot of the engine and and chassis bolts on Norton single models . . . but meant that many of the original Norton fasteners were 'specials', made specifically for a use on a particular Norton model - like these nuts. However, we think making the effort to fit the correct style of nuts and bolts (particularly in stainless that does not rust, unlike originals), makes the difference to a final detailed restoration. I use these nuts myself on all my SOHC engine builds (see accompanying photo).
As a final point when fitting these - even with their reduced hex head, they are a fiddly and tight fit (particularly the rear nuts)- so when fitting, dont rush and make sure the thread is well oiled before fitting - then try and spin them down by hand as much possible first. If you are happy to 'donate' a ring spanner, it is worth considering grinding the outer face of the ring spanner (or socket) so they will fit over the nuts without fouling the castintgs - but regardless, you just need to be careful not to scratch the castings when final tightening, as space is limited!
Originally these nuts were fitted with similar ' Reduced outer diameter' washers . . . and we do sometimes make batch's of these (Item 0011) in stainless steel - but actually - I have found it better to use a special form of stainless spring washer (Item 0102) - as these look just the same when fitted, but also help retain the nuts, if vibration loosens them off slightly. These are used in the photograph of the nuts/studs/washers fitted to a magnesium (pre-war) Manx engine shown in this listing.