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dominator pistons

Submitted by steve_coleman on Thu, 06/08/2020
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Heavy Twins

I am looking for 7.4 to 1 pistons for my 57 dommie 99. I currently have 13325 split skirt pistons fitted which are perfectly ok for use I put the bike to, but I am looking to fit some solid skirt pistons to a spare barrel I have . Can anyone tell me to part number of these hepolite low compression (7.4 to 1) solid skirt pistons fitted at around that time.

Best regards Steve Coleman.

Submitted by lionel_yexley on Thu, 06/08/2020

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Although some people might be able to identify which model Norton you have from the part number it would be best to say whether it is an 88 or 99.

Submitted by robert_tuck on Thu, 06/08/2020

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I have read somewhere that early Dommy pistons were made for a Hillman car , whether this is true or if they did not need the valve cutaways with the early cam  I have no idea.  Interesting though!. Did the UK  make solid skirt low comp pistons or are they going to be some far east or Italian make?.

Submitted by robert_tuck on Thu, 06/08/2020

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You can fit 650ss pistons which have solid skirts , The CR will work out lower than the 650 as the earlier head  pre 1959 gives a lower ratio approx 1 atmosphere.

Thanks Robert, Yes I have fitted BHB 650 pistons in the past and your correct they are have the same distance from gudgeon pin centre to top, therefore the same CR on earller head. the trouble is that they have a shorter skirt and are quite a bit lighter than the original 13325s and on my engine result in out of balance and more vibration. 

Submitted by robert_tuck on Thu, 06/08/2020

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Thats interesting, I have 650 pistons and lightened them  ,knife edged the skirts etc but the motor runs with no more vibration than I remember , It does smooth out more at over 80 mph, not that I go there much. Would be better if smoothed out at 65.

Submitted by david_cooper on Thu, 06/08/2020

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I have some standard size solid skirt 99 flat top pistons. I had a vague plan to use them in  535 Dommie 88.

These are by AE. Box has Ref B/16606. Top is stamped BHB in a circle.

The weight with pins etc is 276 grammes.

Incidentally...I also have a pair of +30 Heppolites split skirt for an 88. Low compression. These weigh 311 grammes. Make of it what you will!

Thanks for that David, My 23270/1 BHB 650 pistons are 275 grammes all up as opposed to 310 on the split skirt pistons, 35 grammes lighter ! The engine must have been balanced on the heavier piston factor. The weight of the split skirt pistons is boosted not only by the longer skirt  but also the substantial Webb's around the  gudgeon pin bosses,   

Submitted by philip_hannam on Thu, 06/08/2020

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The attachment is from the earlier Dominator Service Notes but has been updated as much as possible. Please note there may be inaccuracies in the recorded numbers and that Norton Dominator Cylinder Heads gained higher compression ratios every couple of years. Sometimes due to changes in the head itself but also sometimes due to the piston pin to crown height changing.

Attachments
Ch 41 - Norton Twins Pistons - Updated.doc

Thanks for your reply Phillip, the chart is very useful. The 13325 pistons show CR of both 7.4 and 8.25 , I guess this means production with those pistons ran past 1959 into the fitting of the higher compression head, maybe that's when the breakages started to happen.

The chart shows 19406/19407 as 7.4 to 1 , do you happen to know if these were solid or split skirt?

Submitted by david_cooper on Fri, 07/08/2020

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According to the Haynes manual, there were two piston types, thus two compression ratios before 1960. Then the design changed (head, maybe?) and the same two pistons gave higher ratios. For my 88 I have samples of both, and both are flat topped. But I also have one pair of domed plus a pair of flat ones for 99. Do you have a problem with split skirts? Those on Commando are alleged to be a problem, but not on 88 or 99. Or are they?

Submitted by steve_coleman on Fri, 07/08/2020

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No, I haven't had any problems with split skirt pistons but allegedly some people have, with the crown separating, I don't ride very hard these days. Commando pistons are not split skirt radially, they just have a slots near the gudgeon pin. Are the flat top 99 pistons you have split skirt or solid?

 



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