SOHC E3227 1/2" x 20tpi BSC Nut for Front Frame Engine Bolt - Stainless Steel

Product no.: 0728 E3227

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This is the E3227 0.5" x 20tpi nut that is used on the front downtube engine bolt on Model 30/40 and M30/M40, CS1/CSJ models.  The nut follows original Norton profile - i.e. being a reduced sized Hex and deeper in depth than other commercial 0.5" nuts. The nut is CNC manufactured by us in non rusting stainless steel, with faces linished.

As there were so many variations of Norton SOHC engine bolts/nuts, the table below shows the spec of these nuts and their most normal application :

Norton Engine Nut Attribute Table:


     Attribute Type:  

   Attribute Description                      

       Nut Bore and Thread

   1/2" Shaft.  Thread 0.5" x 20tpi

       Spanner Size/AF Dia

   3/8" Whit Spanner : 0.705" AF Hex.  


       Depth of Nut:

   0.415" / 10.5mm (nominal : +/- 0.5mm)

       Normal Usage:


 Front downtube engine plate bolt nut on

 International and M30/M40 models

 (Items 0726/0727 in our Catalog)

 5          Material:  

 Stainless Steel




 Nuts are flat on their lower face and chamfered

 on their top face.  Linished

Bolt Table Legend and Useful Information: 

AF = "Across Flats', i.e the diamter in inches of hex bar between the two opposite flats - this is the diameter the spanner gap needs to be - we use the correct 'British Standard' bar, as fitted to all Norton single cylinder motorcycles originally, to fit BSW spanners. 

UH = 'Under Head' Bolt Length, i.e. when measuring the length of the bolt shaft - dimensions are taken from the distance at the base of the hex head to the end of the threaded shaft.  It does not take into account the height of the hex head, or the slightly rounded chamfer at the end of the threaded shaft, which are additional.  (Please bear in mind length is nominal - there may  be 1 or 2 mm variation because of batch variation). 

Therefore - when comparing the length shown in the table above to your own engine, you need to take into account the width of the engine casting, engine plates and the height of the nut.  Also, any brackets that you may fit under the bolt head - I have tried to base the length of bolts on not having additional brackets unless I know they were standard (the nuts are normally extra width though and can be narrowed slightly if necessary, or one of the washers omitted

Remember that in some cases, bolts were fitted where there may have only been an engine plate on one side - i.e for the rear of the engine on most SOHC engines, the bolt immediately above the magneto platform has an engine plate only on one side, while the bolt under the magneto has an engine plate on both sides - which is why I make 2 versions of these bolts.  Norton did not bother originally, fitting the same length bolt to both)

Reduced Hex Nuts : for many of the bolts and nuts fitted to Norton motorcycles, the AF (Across Flats) diameter was less than normal commerically available nuts for a given bolt shaft size - the rationale for this was that there was not much room around complex castings, therefore you needed to be able to get access with a smaller spanner.  To allow for this, many of the engine bolts and nuts with these 'reduced head size to shaft diameter' nuts also had a deeper depth of hex as well, to provide as much spanner to hex contact as normal bolts - reducing the chance of 'rounding' the hex when applying a lot of pressure with spanners and providing as much thread on the nuts as possible - but important with the pressure required of engine bolts. 

The nuts shown here replicate the Norton nuts and are deeper than commercially available nuts.  In some cases - i.e. if you are fitting thicker washers or ancillary brackets, you may find the nut is deeper than the bolt allows.  In this case the easiest way I have found to reduce the depth of a nut (if a lathe is not available), is to hold the nut in a ring spanner and place its lower unchamfered face carefully against a linisher (make sure you use suitable safety glasses and gloves when doing this) 

Thread Type:  Engine Bolts on Norton engines tended to come in most of the standard British Whitworth diameters, with the following common thread types, where TPI means 'threads per inch':  1/4" (shaft diameter) x 26 tpi (Threads per inch), 5/16" x 26 tpi3/8" x 26 tpi7/16" x 20 tpi1/2" x 20 tpi.

Most thread profiles were originally BSC (British Standard Cycle), but please note that 7/16" x 20tpi is also a UNF standard thread size and none of the tooling manufacturers will now make the BSC profile tooling - therefore we manufacture this size nut/bolt in UNF profile.  These nuts may fit on original bolts, but can be slightly tighter.

Overall, these nuts are high quality, CNC machine finished to look just as the original Norton items, but in non rusting stainless steel, with original Norton dimensions and profiles.   We linish the hex faces on the bolt heads to ensure crisp finishes and we hope you find the will give that finishing touch to any restoration - or just clean up the look of your oily rag, ride to work Norton!

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