November 2021: A new batch of these popular pistons have now come in and are available again (and updated price is actually fractionally lower than was anticipating).
This is a 80.13mm (or if you have an Inter with a standard bore of 79mm then this will equate to 79mm +44 thou oversize.
They are high quality forged 'slipper' type pistons from top quality manufacturer Omega Pistons, made specially for us - to the same vital dimensions of the pre-war International type Hepolite Ref 5288 piston form - one of these being supplied to Omega as a pattern (see picture for original piston alongside ours). This piston form was the standard higher compression piston suitable for petrol (rather than a 'dope' piston), as fitted to Norton International engines. Although I cannot be precise - the last time I fitted one of these original Hepolite pistons to an International engine it gave a compression ratio of approximately 9.5:1.
As you can see from the photos, the profile of the crown, and its position in relation to the gudgeon pin centre line is identical to the original - and as far as I am aware, this is the first time this pre-war type piston has been available for many years, so you can be confident it should be as per originally fitted.
Fitting Information - Please Read:
Little End Width: This listing is for a piston to fit a Norton International engine - i.e. the piston is designed to take the International conrod which has a Little End width of 1.200"
if you are not sure - check the width of your conrod - original International engines (i.e. aluminium crankcase models) normally had a connecting rod with a width of 1.200" width, while Racing International/Manx (i.e. magnesium crankcase) conrods had a slimmer Little End width of 1.000",
Therefore, this item is for the wider conrod International SOHC engine, if you require the slimmer Manx width, see the seperate Item listing for the Manx type piston - (i.e. Item number 0398 or 0686).
Bore size: This piston is to fit a bore size of 80.13mm - which was the standard (STD) bore size for a SOHC Manx Norton (i.e 499cc rather than 490cc of the roadgoing International) + 20thou. Norton's did this because for the racing models, they did not want to waste any available cc. Because the cost of ordering pistons from Omega in all original bore sizes is prohibitive - we offer it in two variations - either the 79.62mm bore (Item 0396), or 79.62mm + 20thou (i.e. 80.13mm), shown here
As a Norton International Model 30 (500cc) STD piston was originall 79.00mm - this 80.13mm is broadly equivelant to an original 79mm STD + 0.044" thou piston on an International model (please check your original piston and bore), and the barrel should be bored to that 80.13mm dimension. Email us if you require more information on this
Gudgeon Pin To Base of Barrel Height: It is important before ordering pistons for post-war SOHC (and early Longstroke DOHC) engines, to check the distance between the centre of the Gudgeon Pin and the base of the piston crown - as some later engines had a shallower barrel/cylinder head spigot - and to allow for this the piston was slightly different. These pistons have a distance of 34.0mm between the Gudgeon Pin centre and base (i.e. circumference height) of the piston crown. Email us if you require more details.
Piston Weight: Weight of these pistons with rings/circlips etc is @465g (which is compatable with original pistons). If you require information for 'balancing' your crank to this piston (or are just not sure if your crank is correctly balanced), then email us for details on how to do this.
Compression Ratio: The compression ratio we quote here is a guide only, as over the years the SOHC engine was manufactured there were many variations in barrel, head height, valve diameter etc, all of which can affect the compression ratio. However, the Hepolite Ref 5288 piston used as the pattern for our pistons is a good indicative dome for the most normal piston crown fitted to International Model 30 engines running on good quality pump petrol - similar to offered today (i.e. not post War 'Pool' petrol, which sometimes necessitated lowering compression ratio).
It was normal practice when fitting SOHC pistons to measure the swept volume (i.e. stroke and exact bore size) and the combustion chamber volume when the piston is at TDC - and then add or remove compression plates to get to a desired volume. To help with this we can supply nice quality lasercut compression plates in two thicknesses (Items 0275 and 0275b) - but it should be remembered when changing the barrel/head height on SOHC (and DOHC) Norton engines, you also need to check the vertical coupling clearance is still within limits. Email us if more information on this is required, and we also offer a useful MS Excel utility on our www.RacingVincent.co.uk Download page for calculating compression ratios for a given CC to swept volume.