This is the special 13294 (part no listed in 1948 Spare Parts catalog) front engine plate top bolt that is fitted to M30/M40 (pre-Featherbed Manx) magnesium engines. It has the correct 'reduced' head 0.600" AF hex head, with distinctive Norton curved head finish. This set comes complete with correct matching E3224 nut and two washers, all in stainless steel. It is 3/8" shaft diameter with a 3/8" x 26tpi thread
This particular bolt (as far as I know) was only fitted to Magnesium pre-featherbed racing engines - and is very long!, its shaft length is almost 5 1/4" and overall length is just over 5 1/2" long. Because of it's length, oringal bolts are very rare to find - most people instead fit threaded studs with nuts both end
We CNC machine this bolt in-house from stainless steel. As well as having the correct 'reduced head' 0.600" AF hex that was originally used on these bolts, the head shape profile is just as it should be with distinctive shallow dome. It is also fitted with the correct 'deep' E3224 engine bolt nut (sold seperately as item 0667), which allows extra spanner purchase to be applied with less risk of stripping threads than with shallower commercially available nuts.
Sorry this bolt is not cheap - but it's length and head profile means it is a complex and time consuming bolt to make - even on a CNC machine, requiring 4 seperate operations
See this bolt being made on Youtube: https://youtu.be/PHriTiDE5Uc