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Mighty Mica !

Anyone else using mica-insulated sparking plugs ?

I'm making a special effort for the 80th anniversary of the BEF's move to France (and regrettable retreat in the face of 'Fall Gelb')

The KLG KS5 specified by Norton and the War Office is a fiddle to fit as it's 1/2" taller than the M80 'Corundite' which was approved later.

Now that TEL is once again absent from fuel, there is a chance that mica can be run as it was in the 1930s...first impressions are no differences in starting and running from post-war M80s.

Re: Mighty Mica !


I have been using KS5 sparkplug since the first time I kickstarted my Norton after the restoration! I also replaced a BERU plug that I had fitted to my civilian WD16H with one of the spares KS5 found in the toolbox!

More than 2500 miles on the clock and still no worries so far!


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Re: Mighty Mica !

That's good to know, Kostas. I hadn't realised. What sort of fuel do you get in Athens ? Standard E10 ?

Decent 18mm KS5s are quite a bit harder to find than the 14mm L777s...the attractive brass cooling fins also means that they are more likely to end up with collectors.

It is perhaps no longer a matter for discussion but it was for many years accepted that mica insulation was problematical.

In his 1991 booklet "The Vital Spark", published by The Rolls Royce Heritage Trust, Keith Gough (ex-BAC, KLG and Lodge engineer) illustrates the fouling and pin-hole attacks on mica caused by tetraethyl lead. This was first noted as a problem in aero-engines during the mid-1930s and led to the Lodge 'Sintox' and KLG 'Corundite' sintered aluminium oxide insulators.

Initially, wartime 'pool' fuel for vehicles was unleaded but the changes in fuel supply arrangements prior to Normandy meant that it appeared in B-vehicle supplies too. At that point, mica-insulayed plugs must have fallen out of military use.

The relatively recent removal of TEL from pump fuel probably means that we can once again use mica plugs as reliably as during the 1930s.

Re: Mighty Mica !


There was a good mica KS5 with very little wear in my 16H when found. I dismantled and clead it and it used to run fine on unleaded fuel. Now with the new E5 fuel it misses on the pickup from low revs when the engine is under load, no problems onece the revs have picked up. I now run with a M50 that works fine. Pity as i like the look of the old KS5 with the copper cooling fins. As you say a collectors plug now.

Tim W

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