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Ring gaps and piston clearances.

Submitted by kevin_feltoe on Wed, 13/04/2022
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Light Twins

Having aquired a pair of second hand +30 pistons that are somewhat better than the ones I've been running for longer than I care to remember, I'm close to the point of putting the engine back together. The barrels seem to be in good order and I'd prefer not to have them honed.  'I've heard various stories about the only place near me that does that work'. Can anyone suggest what the minimum clearance ought to be between pistons and barrels to hopefully prevent a seizure. I'm afraid that my best efforts at judging the clearance involves feeler gauges, (I know that isn't too professional), and I've discovered that 0.03" to 0.04" seems to be the general standard. Also, what size ring gaps should I be setting? 

All advice will be much appreciated.

 

Submitted by steven_tyrer on Wed, 13/04/2022

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I think you have missed a decimal place out as 0.003" to 0.004" sounds about right, from my Navigator manual I have the following for the ring gap.

Remember that the pistons are oval shaped

 

Navigator ring gap

 

Submitted by william_dunn on Wed, 13/04/2022

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The above information as provided is correct. I'm unsure why you do not want to hone them and suggest that this may lead to a glazed bore and the rings wont seat in properly.

You didn't mention, are the ring sets new or old and are on the used pistons you want to re use? If that is the case it's not something I would do.

Suggest you take the barrel to an auto engineering firm and have the bores and pistons measured properly, if all is  inspect after honing fit new rings that are gapped as per manufactures specification. It may seem like a lot of extra steps and an increased cost but a top end offer haul is significant and if not done properly a real waste of money and effort. Just my 2 cents worth, best of luck.

Submitted by kevin_feltoe on Wed, 13/04/2022

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I've spent some time worrying about the very things you suggest William. However, here in Norfolk we don't seem to be too well served by auto engineering firms that can do that work even though they call themselves auto engineering firms. A guy at the last one I went to referred me to his dad, who referred me to another firm, with the immortal words, 'they are c--p'! It seems they had messed up his Brough side valve barrel.

The rings I'll be using are new. That's why I wanted to know the gaps. Oh to live in the midst of civilisation! I guess I'll have to widen the search.

Submitted by david_cooper on Wed, 13/04/2022

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Honing doesn't necessarily mean removing a significant amount of metal. It's about getting a fine cross hatch finish to retain just enough oil to lubricate the bores. Otherwise (as Willian says ) shiny bores might well give you lots of blue smoke and oily plugs...you can do it yourself but remember to wash and scrub literally with hot soapy water afterwards to make certain there is no abrasive left on the surfaces.

 



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