350cc (SOHC) Inter Model 40 Hepolite Piston - Special Order (Approx 7.5:1 CR)

Product no.: 1006 A10/160

In stock
can be shipped within 120 days

£0.00
Recommended Retail Price, plus delivery


We have been asked by 350cc SOHC customers in the last 5 years - if we have any plans to offer a 71mm/350cc version of our Omega slipper type piston?  Well, sorry to say, this is not likely to happen in the near future (this update being March 2021) - as the cost of setup and 'Master' for the original 350cc 'slipper' type Norton Model 40 piston is prohibitive, given the limited number of customers asking for them.  

With all that said - we have been looking at alternatives - and this includes looking at the range of pistons the current owners of the 'Hepolite' piston brand name are now making.  Having spoken to them, and looked at their range of pistons available for other British motorcycles - I have found that they offer a piston (originally to fit a British parallel twin) which has most of the vital dimensions in common with the roadgoing 'lower compression' version of the Model 40 350cc SOHC piston - as shown in the photographs here, with the Hepolite piston 'as supplied' on the left, and an original Norton Model 40 350cc piston (lower road compression - possibly around 7.5:1) on the right.

As you can see, the two most important dimensions are almost identical - this is a 71mm bore piston (the STD bore for a 350 model Inter), and the height from the centre of the Gudgeoon pin to the base of the piston is virtually identical.  There is also a 71mm + 20 thou version offered, although they are not curently in stock (possibly for the duration of Covid lockdown period). Note:  the Hepolite STD piston is actually stamped +40 thou, as the original motorcycle engine they were intended for has a slightly smaller standard bore - but they offer this piston in +40 and +60, which equates to SOHC Model 40 (71mm) STD and +20 sizes.

However there are limitiations to using this piston, which need to be considered and are listed here:

1. Pistons are cast, not forged - their design looks very similar to Hepolite 1950's design with full skirts and milled slot under the ringland.  Therefore I would not recommend them for racing or continued maximum speed work (if anyone still does that on their 350 Inters!).  That said they do look very nicely CNC machined from low expansion silicon alloy - and looking underneath (see photo) - there are casting webs around the Little End area that reach out to the cylinder wall skirt area.

2. They are only available in two 350cc (71mm) Bore sizes - STD and +20 thou:  Sorry, but if you have an engine that is already out at +40 or +60 you would have to consider an alternative, or having your cylinder re-linered.

3. The Gudgeon pin diameter is fractionally smaller than the standard Norton 0.875" diameter: The Gudgeon Pin diameter on these pistons are 0.750", while Norton 350 Gudgeon Pin diameter is the same as the 500cc model - 0.875".  Therefore, if we are to supply this piston, you would need to accept that (and make your own decision if this will affect reliability - although it is highly unlikely - as the BSA B50 piston was often used in 500cc Norton's when available, for racing, but they were fitted with smaller gudgeon pins). Therefore, for anyone wanting to order these pistons, we can also supply a replacement Little End, to fit standard SOHC con-rod, but with a smaller 0.750" internal bore. Incidentally - the weight of the piston complete with Gudgeon Pin, rings, circlips is @355 grams, which is relatively light - so I would imagine the slightly heavier Gudgeon Pin + piston would be comparable to an original Norton 350 piston setup.

4. Con Rod Width:  A Norton 350cc SOHC Model 40 conrod has a 1.200" Little End Width.  The engine these Hepolite pistons were intended for are 1.125" - so the piston will need to be modified by milling an additional 75 thou width into the Gudgeon Pin cast area.  Again - we are looking to supply these pistons with this modification already made, as part of the cost

5.Valve Cutaway: As manufactured by Hepolite, the pistons have the correct crown to gudgeon pin height (the difficult dimension to find), but the valve cutaways are not big enough for 350cc SOHC engines - therefore these would either need to be machined by the customer for their own engine spec, or again - we can mill these for you, to the same specification as the original Norton piston shown in the photograph.

So if you own a SOHC Norton 350cc Model 40 - unfortunately we can supply a forged piston 'off the shelf' - but there is at least a viable option available.  We do have a small stock of these pistons in stock - but have not manufactured new Little Ends yet, or modified the pistons as above. 

If you wish to enquire - either to purchase a piston as supplied by Hepolite, or for a quote for us to modify one as above, then email us on sales@racingvincent.co.uk, - quoting Item 1006 - Hepolite Model 40 piston. 

As at March 2021 - these are Special Order, but would expect the RRP of piston, supplied with modifications to gudgeon pin width, revised valve cutaways and new Phospher Bronze Little End is likely to be @ £220 - £240.

 

 

 

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