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Nominal Piston / Bore sizes Electra

Submitted by trevor_collier on Sun, 19/01/2020
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Light Twins

I am slowly working through some issues with the Electra a bought around a year ago and have moved on to looking at why it smokes quite badly when running. Having taken the barrels off I can see a few issues, for example the piston rings on one side appear to be upside down and there are random scratch marks on the bores that look like somebody has been trying to clean then up with emery paper.

I can sort that but I want to see if the bores are any good and what size they are. There are no makings on the pistons to go by and I want to at least put new rings in when it goes back together. 

Does anybody have the nominal bore sizes for standard bores and the various oversizes so I can measure them to find out what I have. 

Submitted by robert_tuck on Sun, 19/01/2020

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If the motor was quiet  and no measurable vertical scores in the cylinders I would be just fitting some new rings.  The light use that classics get will give years of use.

Submitted by trevor_collier on Sun, 19/01/2020

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Robert, That's what I am planning to do once I can figure out what size rings I need, to do that I need to be able to tell if it is standard or an oversize bore which is why I asked the question.

Submitted by john_holmes on Sun, 19/01/2020

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Measure the bore, if its 66mm to 66.1 then its std, every 10 thou is .25mm so get 66.25 to 66.35 and its 10 thou over, 20 thou is 66.5 to 66.6 etc. 

Submitted by richard_cornish on Sun, 19/01/2020

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Hi Trevor,

   The best way to see how much bore wear there is would be to remove each piston from the con-rods and insert  them into their respective bores with-out rings. A feeler gauge inserted from the bottom of the barrel with the piston half way up the stroke at either front or back of the skirt should measure around 0.006" to 0.010", much more than this will need a re-bore. There will also be a pronounced ridge at the top of the barrel after removing any carbon.

   It will be unusual if any oversize is not stamped on the top of the pistons, and if not it is probably standard 66mm. If you don't have an internal micrometer gauge you should be able to get a fairly accurate measurement with a steel rule across the unworn area at the top of the barrel.

Submitted by trevor_collier on Tue, 21/01/2020

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Thanks for your responses all. I have now establisheed the engine is still on standard bores (that was a suprise) and after a good clean up with the honing tool the bores look to be OK. I will try rebuilding with a new set of rings, once I have removed all of the excessive blue sealant that is around every jointing face!

 



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