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Ring end gaps and rebores

Submitted by mike_haworth on Tue, 19/03/2019
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Heavy Twins

On 1959 Dommi '99' - I've purchased a replacement barrel said to be 0.020" oversize. However, checking the end gap on a new + 0.020" ring it is around 0.030". Acording to the Norton Maintenance Manual this should be in the range 0.008" to 0.010" and to the Haynes Manual 0.018" and 0.020" . Either way the gap is bigger than specified. Does this mean I should go for a rebore up to + 0.040" and buy the appropriate oversize pistons and rings ?

As I expect everyone will say, "go for a rebore" ................. Given that I live in Dorset, any recommendations on where to go for a rebore ? How should I specify the size as 68mm + 0.040" or 2.716" ?. How much should I expect to pay ?

Submitted by derek_colville on Tue, 19/03/2019

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Hi

I had a similar situation with my '52 model 7, rebored to +0.040". The book says the ring gap should be 0.008 - 0.010 but it was actually around 0.030. As they were brand new pistons (of Italian origin) and the rebore was done to match the pistons I have taken the view that the new rings are intentionally manufactured to produce that gap of 0.030 so have fitted them. I have yet to run the rebuilt engine so can't confirm if all is OK at this stage.

Hope that is of some help.

Derek

Submitted by richard_hudson1 on Tue, 19/03/2019

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Hi Mike,

Does the bore in your new barrel look new or has it been used since it's rebore?

If it is new check the bore with an internal micrometre or vernier, it should be 68.5 mm dia. and if it is it is the rings which are incorrect. If the ring you have tried is correct then the bore will need to be 68.66 mm dia. to give you the +0.030" end clearance you are reading.

if the bore is 68.5 mm dia. just go ahead and use the rings, the extra gas escaping through the larger gap will be insignificant.

Regards

Dick

Submitted by dan_field on Tue, 19/03/2019

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You don’t say where you are in Dorset, if it’s West Dorset, take it to Dave Massam at Colyton, he’ll measure it for you and can do the rebore if it needs it. He’s an excellent engineer, and even has a Norton! 

Submitted by robert_tuck on Wed, 20/03/2019

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With the scarecity and cost of barrels I would not be in a rush to rebore. If the piston clearance to bore is reasonable then  its  likely that some rings can be found that will do the job. It has been said that the ring gap in itself is not critical. more to the point do the rings exhert enough pressure on the bore to run in and seal.

Submitted by ian_soady on Wed, 20/03/2019

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... is much better than one that's too small for obvious reasons. I'd probably live with the .030" gap.

Hi Dick,

           Yes the bore in the new barrel has been used and is clearly worn. However, I've tried a + 0.040" ring in it and there is no gap i.e. it wont fit in the bore at all. So, unless I can get rings that are bigger than + 0.020", but smaller than + 0.040" I'll never get the gap as specified without a rebore. This said, from the replies I've recieved most don't see this as absolutely critical.

Regards,

Mike

Submitted by Xanna_jeannett… on Thu, 21/03/2019

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now  Norton piston ring end gap should be no more then 8 thou and 9 thou of the oil ring end gap  bore size  should be no more then 40 thou over size  after this a new set o sleeves  there is a reason for this,   

haynes has  wrong information  

      yours  anna j    

 


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