This is a 79.62mm (500cc) forged slipper piston which copies the same vital dimensions as the original 1940's-50's Norton Heplex 572 OHV (Model 18 and ES2) piston, other than having a slightly raised crown - which on most OHV models should relate to an estimated circa 8:1 compression ratio. As with our SOHC slipper pistons - these are the highest quality forged slipper pistons made by top manufactuer Omega.
Price is per piston - piston includes rings, Gudgeon Pin and circlips
As can be seen in the accompanyning photographs - original post war (Heplex) OHV Model 18 and ES2 pistons were most often of the full skirt and flat dome variety - as per the original Norton OHV piston in the photo (taken from a late 1940's standard ES2 many years ago).
The most importand dimensions which we have copied into these Omega pistons are the correct Gudgeon Pin Diameter, the width of the OHV/SV con rod Little End distance and most importantly - height from the centre of the gudgeon pin to the base of the piston crown, and with this, the same base piston crown chamfer as the original Heplex Model 18/ES2 piston.
However, we have had these pistons made with a slightly raised crown in the centre - raised by 10mm at the centre, which should provide a compression ratio of approximately 8:1 (as original pistons and motor specifications in the 1940's were more suited for the lower quality petrol available at the time). As can be seen - the valve cutaways have also been enlarged slightly to allow for this.
Norton Side Valve 16H piston usage: It should be noted that the WD side valve 16H Norton, and the later Postward 16H models, all used a piston similar to the ES2, but this piston also had a raised crown, to achieve a similar compression ratio to the OHV model in the larger SV combustion space (but of course, original 16H pistons did not have the need for valve cutaways and were normally of a more heavyweight skirt construction for long life. The construction of these Omega pistons may make them lighter than original 16H pre-war pistons (although post-war type were lighter and comparable) - but are a viable alternative to be considered for 500cc 16H sidevalve models. A picture of an original WD sidevalve piston is shown in the accmpnanying photos for comparison
Norton CS1 (SOHC) Usage: These pistons are similar to the higher compression 'International' pistons we also offer (Item 0396 and 0397), but with slightly different dome dimensions (and of course, a lower compression ratio than the International type). If you have either a Walter Moore or Arthur Carroll type CS1, and are looking for a lower compression version of our International piston, then email us on email@example.com to confirm if these will fit your engine
As a final point - although most people associate 'Slipper' type pistons with racing engines - but for all uses, they are a high quality forged piston that have a steady and reliable expansion rate and are very stiff and rigid in construction. And surprisingly - the total 'touching' side wall of the thrust face of these Omega pistons is actually fractionally more than the original Heplex 572 Norton piston shown in the photograph.
Fitting Pistons: As with the fitting of all pistons to older engines (which may have been previously modified) - we strongly advise you satisfy yourself there is sufficient piston to head/clearance in your own engine when fitting one of these pistons - I always put a thin layer of plasticine on the top of a piston crown - then slowy turn the assembled engine over by hand first - there should be at least 60 thou of clearance on valve cutaways and to ensure the piston is not touching the head at any point. a Digital caliper, using the depth gauge prong into the plasticine is ideal for this.
Piston Weight: These pistons weigh 450grams complete with Gudgeon Pi, rings and circlips - which is within a couple of grams of the weight of the original Norton Heplex 572 piston (unlike other non forged modern OHV substitutes, which are considerably heavier). Therefore the balance charateristics when using these pistons should be comparable to original specification
Little End Width: This listing is for a piston to fit a Norton OHV/SV or CS1 specification engine - i.e. the piston is designed to take the Norton conrod which has a Little End width of 1.200"
Bore Size Information:
Original OHV and SV Norton (Longstroke) 500cc pistons had a standard bore of 79mm, and were then offered in standard oversizes of +20, +40 and +60 thousandths of an inch. Occasionally +30 thou oversize can also be found.
We can currently only offer these pistons in two bore sizes - 79.62mm (which is this Item listing) or 80.13mm (Item 1062)
79.62mm in OHV/SV oversize parlance can be converted to 79mm + 24thou, while the 80.13mm version can be converted to 79mm + 44thou. Hopefully one of these two 79mm oversizes will be suitable for your engine (or of course you can consider re-lining your cylinder if already out at +60. If based in the UK we can recommend people we have used for this in the past).
When taking your barrel to be bored/honed, ask for the barrel to be honed to the exact size shown on here (79.62mm or 80.13mm), Omega will have worked out the correct clearances for these pistons for that bore.
In case you are wondering why we have chosen these two bore sizes for our pistons - our original Omega forge patterns were made for the Manx Norton piston size of 79.62mm and one oversize of this - as original Manx Longstroke pistons were a slightly larger bore, to allow the full 499cc capacity to be used for Grand Prix racing. These two sizes also allowed a useful 'oversize' piston for standard 79mm engines.