500cc (SOHC) M30/Manx Piston: 79.62mm High Comp/'Alcohol Dome' - Slipper Type (Approx 12:1 CR)

Product no.: 0686 A11M/160A

In stock
can be shipped within 120 days

Recommended Retail Price, plus delivery

This is a 79.62mm bore (500cc) forged slipper piston to fit Norton Racing International (i.e. M30) and Longstroke Manx 79.62mm x 100mm Longstroke singles - this version is a very high compression type meant for alchohol/methanol or racing fuels (not for petrol) and for competition use only

They are high quality forged 'slipper' type pistons, made by top performance piston manufacturer - Omega Pistons.

This listing is for the very high compression version of our piston - designed to run on methanol or alchohol with a very high dome, which will provide a much higher compression ratio than are other 'petrol' type pistons.  It is of the M30/SOHC Longstroke Manx type - i.e. the piston is designed to take the Manx type conrod which has a Little End width of 1.000" (if you are not sure - check the width of your conrod - original Racing International/Manx conrods had a Little End width of 1.000", while International was 1.200". We only supply this 'methanol' version piston to fit the 1.000" width conrod, but you can mill the inner gudgeon pin faces yourself if you wish to fit it to the Inter type conrod - not a difficult job for anyone with a vertical milling machine.

As you can see from the photos, the profile of the crown, and its position in relation to the gudgeon pin centre line is much greater than that of the normal Hepolite high compression International petrol version, and is designed to give ultra high compression for use when using alchohol fuels like methanol - for racing purposes only.  For comparison - see the photograph that shows this piston alongside our Omega 'petrol' piston.  Do not purchase this piston if you are intending to run your engine on the road, using pump petrol fuel - for that see instead the listings 396/397/398.

However, this piston could also be used for other high octane racing fuels - providing you know the optimum compression ratio with other fuels, and have measured the compression ratio, once fitted.  It is always possible to add compression spacer rings (which we offer) - but remember with SOHC engines - this may also entail amending the top Oldham Coupling height (we can also offer varying thickness couplings)

We have copied the crown of this piston from another period methanol competition piston, designed to fit into the Norton SOHC Longstroke Manx type cylinder head hemisperical cylinder, with valve cutaways to accomodate most SOHC racing valve sizes (and is sided - with the inlet valve cutaway slightly larger than the exhaust valve cutaway - ensure you check this when you fit the piston)

It is difficult to be specific about the compression ratio - as it will be dependent on the specification of the engine you are fitting it to - crankcase height, barrel length, spigot length will all play a part, but I would estimate ratio to be somewhere between 11:1 - 14:1 dependent on your engine spec and barrel spacers fitted. 

Note: As with any racing engine - it is very important that if you are intending to use this piston you must satisfy yourself that when fitted, satisfactory clearance to the valves at full lift and cylinder head are achieved.  We recommend a minimum of 0.060" (60 thou) clearance should be achievable at any point on the piston crown or valve cutaways (at full cam lift).  Use plasticine or similar to check on assembly before running the engine.  If the necesaary clearance is not available on first fitting then add suitable barrel spacers until it is achieved.  Remember as well, that by altering the barrel height to alter compression ratio you may also need to amend the vertical shaft assembly length - to help achieve this we also sell barrel spacers (item 0275) and different length Oldham Couplings (Item 0684).

And finally, if running on methanol/alchohol fuels - you will also need to ensure your carburettor jetting is setup correctly as well.  As an indication (indication only! - it varies by engine), it is not unusual for a 500cc Inter/M30 engine running with an Amal TT/RN carb on methanol to need a main jet between 900 - 1400, compared to @450 main jet on petrol.  Likewise an Amal pilot jet of 120 will be required.

We hope that for anyone already racing classic motorccycles everything we have said here will be second nature and that when running alchohol fuels it is fully understood that clearances are a major concern and needs careful checking - but if you have any doubts about this process we advise you not to purchase this piston.  With that in mind we re-iterate our standard disclaimer that although we ensure our parts are manufactured to the highest possible standards - due to the nature of usage, we can not take any responsibility for issues caused from usage of this piston - responsibility of setup is entirely with the purchaser.  Hope this goes without saying, but we are saying it anyway!  We use these pistons in our racing engines but - go through exactly the same checking process on every engine build - if need be we will alter engine configuration or even amend valve cutouts to suit the engine specification on build, so bear in mind you may need to do so as well!

Weight of piston with rings/circlips etc is @495g

Bore size: This piston is to fit a bore size of 79.6mm - which was the standard (STD) bore size for a SOHC Manx Norton (i.e 499cc rather than 490cc of the roadgoing International . . . the racing bikes did not want to waste available cc!). If you are fitting this piston to a Norton International : then 79.62mm equates to International 79mm STD + 0.025".  When boring your barrel it needs to be finished to : 79.62mm 


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