E4251 - SOHC Mod 40M (350) Wasted Head Rear/Short Engine Bolt Set (3/8"x26 tpi) - Stainless Steel

Product no.: 0723 E4251S

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This is the E4251 3/8" x 26tpi above magneto/rear engine bolt set as fitted to early Norton Model 40M/Pre 48 Manx Model 40M competition models - as fitted above the magneto, with the competition type distinctive Norton type 'Wasted Head'/reduced hex curved head profile.  The set consists of hex head bolt, deep nut and two washers - CNC manufactured by ourselves in Stainless Steel. 

The engine bolts fitted to 350 SOHC engines are a bit more confusing than the 500 versions, because there were commonly 2 sizes of crankcase - the earlier type as shown in these photographs being physically smaller than the 500cc versions.  However, in 1948 the 350 and 500 crankcases were standardised on the larger 500cc crankcase casting.  

I do not profess to be an expert on all the 350 variants, but know that on many of the smaller/earlier type 350cc crankcase casting, the rear 10pm and 11pm (above and below magneto platform) were of 3/8" shaft diameter - rather than the 7/16" shaft version fitted to 500cc and later 350cc crankcases (0718/0719 and 0724/0725).

So please check your crankcase to be sure, but if you a (350) crankcase with 3/8" shaft bolt diameter in the 10pm position (i.e. just below the magneto platform), and wish to fit the roadgoing 'rounded' head, then this is the bolt you need.

Note 1: In the 1948 Norton Spare Parts catalog the E4251 bolt is shown as fitted to the earlier 40M (early 350 Manx) crankcase, which is this item.  We have made this with the pattern of 'Wasted Head' which is often seen in period photos of that period - i.e. the head is counterfaced to remove weight as much as possible.  It also looks very pretty.  We also sell the same bolt with the roadgoing 'rounded head' profile, more commonly fitted to roadgoing models  - which is Item 0720/0721.

Note 2: Norton look to have fitted the same engine bolt both above and below the magneto platform (i.e. 11pm and 10pm).  Because there was only an engine plate on one side above the magneto, I offer this bolt in 2 lengths, this being the slightly shorter version fitted above the magneto platform.

As there were so many variations of Norton SOHC engine bolts, the table below will help assist you confirm if you are ordering the correct bolt, and you may find this bolt will also fit your bike model for another application :

Norton Engine Bolt Attribute Table:


       Attribute Type:  

   Attribute Description                      

          Model Type:

   Early (small crankcase) 350 Models?



 350cc/Model 40M and SOHC Manx models


         Bolt Shaft Diameter:

   3/8" shaft diameter

         Bolt Shaft Length:

    4.040"/102mm (UH to end of thread length

         Head Type:

   Norton competition type 'Wasted' head

         Spanner Size/AF Dia

   5/16" Whit Spanner : 0.600" AF Hex.  


         No req'd per engine:

   1 per engine

         Bolt Radial Position:


 Approx 11.00pm - above magneto

 (if crankcase is clock face from timing side)


         Approx Nut Depth:

 10          Material:  

 Stainless Steel


        Thread Type:

   3/8" x 26 tpi


   Kit consists of hex head bolt, 2 washers, nut.

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)

'Rounded Head'/Reduced Hex Head : Norton's used a distinctive rounded dome on many of their larger engine bolts - these are replicated exactly on our bolts, as the photographs show.  This style of head was used on many Norton bolts of the '30's-'50's. 

In addition many engine bolts had an AF (Across Flats) diameter that was less than normal commerically available bolts 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 with these 'reduced head size to shaft diameter' bolts 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.  We have replicated these deeper heads, with reduced hex diameter to shaft as per the originals, not only giving that authentic look, but ensuring the nuts .  To our knowledge this type of bolt/nut is not  had, as well as a smaller hex (reduced head) than is normal for a commercially available 7/16" bolt.  They are very distinctive and I always struggle to find these when restoring or building a project Norton.

Bolt Radial Position In Relation To Crankcase:  In the table above I show the radial position of the engine bolt listed here, in relation to the crankcase, if looking on the Timing side of the crankcase as a Clock Dial.  Therefore, the front engine bolt closest to the front down tube and on the horizontal datum line with the mainshafts would be - 3pm, hope this makes sense!  I expect most owners used to Norton single engines try and work out in their head the relative positions of engine bolts in the same way. Clock times are approximate!

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.

In most cases, orginal 'New Old Stock' Norton bolts were used as templates for these bolts - although being practical - it is very hard to tell what is absolutely original or nut for each engine! - there were so many bolt variations and lengths over the years, with some engines seemingly using different lengths and shaft diameters for the equivelant engine by the year.  It is worth noting particularly that pre 1948/49 350cc cranckcases are physically smaller than the equivelent 500cc crankcase - but they standardised them in 1949

Therefore - please check your own engine to check which ones you require before purchasing, and double check the fit before final tightening - in case you need to return.  We hope you understand, we will not accept returns if you have marked the finish of the hex or shafts as we will not be able to resell.  Email us on our normal sales@racingvincent.co.uk if you need any more information before ordering.

Overall, these bolts are high quality, CNC machine finished engine bolt sets manufactured 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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