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Correct plug gap

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Correct plug gap

Posted by George Phillips at September 14. 2018

Looking through past posts there seems to be a variety of opinions on the correct plug gap for a mid fifties ES2 ranging from 15 to 25 thou.

One "Book of the Norton" 6th Edition 1951 states 15 - 18 thou but 15 thou recommended. A later 8th Edition 1957 that I have states 18-22 thou. Then the 9th Edition 1963 states 20-22. A further Maintenance Manual - undated but cost 2/6d at the time and looking like mid fifties - states 15thou for a KLG F70.

James Smith's excellent manual "Classic M/cycle Electrics Manual" notes that modern plugs are supplied "pre-gapped" and "it is preferable not to change this."

Does modern petrol affect the plug gap?

Also, the CR increased throughout the 50's from 6.45 through to 7.1. Would this have a bearing on it?

So where does that leave us? Trial and error and lots of plug washers?

I think on balance I'll go for 18 (sitting on the fence). Any views?

George

Re: Correct plug gap

Posted by David Cooper at September 14. 2018

Generally they advise lower for a magneto (15) and higher for coil (25).

I don't believe I've ever been craftsman enough to confidently distinguished between numbers as close as 15 and 18...especially as at small gaps the side electrode is not parallel to the tip.

 

As for modern plugs being ready gapped - I'm fairly sure the ones I buy are usually at about 25, which is fine for my electronic+coil bike, but needs to be closed down for the magneto.

 

I'd suggest posting on electrical, and then Al Osborne is more likely to see it.  I don't believe the fuel is anything to do with it.  As I understand it, a bigger gap means a bigger voltage build up before the sparks strikes, so gives a fatter (more energetic) spark, but allegedly it's more likely to overstress the HT insulation in a magneto.

Re: Correct plug gap

Posted by George Phillips at September 14. 2018

Thanks David. That all makes good sense.

I've added the post to Electrical.

 

George

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