This is the distinctive oil level filler plug as fitted to all Laydown Norton Gearbox's (under and to the left of the kickstart boss) from 1951-56. As per the original it has the correct extra fine thread, square head, with screwdriver slot. Ours are CNC machined in Stainless Steel and also look vgery pretty.
As you can from the photogrpahs, I have shown pictures of the original Norton Gearbox laydown outer cover, that shows where this oil level plug resided - and as its name suggests, it is there to give owners an indication when the main gearbox casing had been filled with oil to the correct level. You would first drain out old oil from the main gearbox sump plug, and once having replaced that you would unscrew this distinctive plug and then place new gearbox oil via the oval shaped clutch worm inspection cover. Once the correct oil level had been reached, oil would flow along a special oilway at the back of the inner gearbox shell into an area behind the oil level plug. Once any excess had drained, the plug was replaced and you knew the oil level was correct.
I have seen 3 versions of this plug - one with just a square head (which is actually very difficult and fiddly to remove, because few owners have a spanner to fit the square head and resort to a large adjustable that hits and scratch's the gearbox cover), and then the type with a screwdriver slot parallel to the outer square, and finally the most common type you see here - where the screwdriver slot is diaganally across the square - which is the best type (and I assume the type that Norton themselves quickly settled on). This allows for a screwdriver to be used to loosen the plug, while also offering a longer screwdriver slot length than the type where the slot is parallel with the square.
We replicate this type exactly as per original - but we CNC machine these from stainless steel and as well as the fine thread and flat rear face, we also perform two milling operations - we mill the square head, then mill the screwdriver slot at 45 degrees to the square in stainless steel. So as well as being very practical (particularly as most originals are now badly chewed) they also look very pretty and do not rust. (this is not the cheapest of fasteners, but the time taken to perform two milling ops as well as the turning reflect this).
This particular screwplug is particularly unusual in that it has an extra fine thread (often mistaken for 3/8" BSC - but it is actually finer and a bigger diameter than BSC - so dont attempt to fit a BSC bolt by mistake!). However, it also has a particularly flat end to the threaded section, with almost no thread chamfer - where it locates into the gearbox filler area (see accompanying photographs).
The original 1955 Norton Spare Parts List also lists a washer for this plug (Part No 13833), but does not include a picture. I vaguely remember stripping one of these gearbox's many years ago and finding a small piece of brown gasket paper fitted to the end of this flat threaded area - which would make sense . . . but it was so long ago I cannot be sure, so I will leave you to decide if you want to fit one.
Final point - if you are considering buying this plug, but also considering changing any other screws on the outer gearbox cover - then please look instead at Item 0996 - as that kit contains all the cover cheesehead screws, bottom blind nuts and this filler plug within that kit. Also, If you really want the type without the screwdriver slot (i.e. because you have a special square spanner, we did make a handful without the slot - drop us a note when ordering, if you want one of this type instead)