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#841636 03/02/21 3:02 pm
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Around when did spark plugs found on motorcycles change from 18mm to 14mm? The 18mm size is found on all of my 1920's bikes, and on my '39 Francis Barnett, but my '35 Royal Enfield has a 14mm.

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Around the time they figured out bigger valves were important ! laughing


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Ha! Mine are mostly two strokes, we don't need no stinkin valves! smile

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18 mm plugs were used by some manufacturers (perhaps not British) even well after WW2. I guess the change came gradually over decades.

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As a broad generalization, the larger diameter spark plugs were employed in older engines where fouling was a problem. Fouling isn't a major concern in the newer engine designs so the spark plug diameter doesn't need to be as large. Also, as noted above, larger valves and more of them is pushing the plug diameter to become smaller and smaller.

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Thanks all, interesting thoughts on the subject. Most of my bikes with 18mm plugs are two strokes, the only one is a 1929 Douglas which has special "pockets" in the head to help prevent fouling. I guess the combination of the 18mm plugs and the "pockets" is sort of a belt and suspenders thing. It turns out that the 1935 Royal Enfield with the 14mm plug had an 18 to 14 adapter in it, so it too is designed for 18mm plugs.

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The changeover was indeed mostly in the mid 1930s.
Royal Enfield changed about when most others did,
my 37 Enfield has a 14mm.

So it neatly lets you date cylinder heads to either early 1930s (18mm)
or later 30s (14mm).

The same could be asked about 12mm plugs.
And have I seen some 10mm ones ??

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My '39 Francis Barnett has 18mm and my '43 BSA M20 has 14mm so maybe that narrows the range a bit, but I would also think it would be up to when the various manufacturers made the change, as Pelle said.

Speaking of 12mm plugs, when I last purchased some 18mm plugs, the store accidentally gave me a very odd 12mm plug in with the order, the threaded portion of it was over 1.5" long. I had no use for it, but it sure was interesting to imagine the cylinder head it was designed for.

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Originally Posted by cycarmark
My '39 Francis Barnett has 18mm .

Thats quite late for an 18mm plug ?
Either they had a big stock of cylinder heads already,
or they didn't sell many of that model !

I'd be curious what had 18mm postwar ?
Airplane engines excepted.

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Originally Posted by Rohan
I'd be curious what had 18mm postwar ?
Airplane engines excepted.
Petrol-kero tractors. Our 1948 Inter AW-6 had 18mm plugs.
The pilot motors on Caterpillar crawlers & dozers might have as well.

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I have an ILO-engined Monark, early 1950's. A 150 cc two stroke with 18 mm plug. I'm assuming there were others, but I don't know much about early postwar two strokes.

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As the spark plug was a French invention, and we know what sort of sticklers for originally the French are, I would hazard a guess that untill the patient for the original plugs expired any one who made them had to make them exactly according to the original French design.
Once patient had expired then plug makers were free to do what they liked.

And remember most of the early engines were side valves so no space constrictions to worry about


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Wait a minute, while a lot of folks had a hand in it, it was Robert Bosch who had his name on the patent. ?
And Champion. Lodge and AC were in there pretty early too.

I wonder what was actually patented ?
And how there were soon so many manufacturers.
Engines had hot tubes prior to that, so was but a refinement to make a momentary fire, rather than keep the fire alight ??

And the magneto was a separate invention, as late as 1906 or thereabouts ???

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As you know Rohan I am always happy to be proven wrong.
AFAIK the spark plug was invented in France by Edmond Berger and up till WWI France made the bulk of the worlds spark plugs
The other factories made them under license which was quite restrictive
This is why they are all metric threads .
Just have a think, would any USA maker sell their soul to the devil and make a metric item is they could avoid doing it ?
Champion will tell you it was Albert Champion who invented the "Modern" spark plug although there is no definition of "modern".
Bosh just mention that Robert had the first patient for a "successful" plug again with no definition of "successful" .
Then of course there were patients on the machines & processes used to make spark plugs.


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According to the sources, Mr Berger didn't patent it.
Mr Bosch did.
A very large number of years later.

The French probably needed lots of plugs because they made lots of engines !
De Dion Bouton reportedly had sold 40,000 of their engines before anyone else had made much of anything at all.

As someone once quoted, the Germans invented motorcycles, the French perfected them and then the English marketed them to the world.
Which is a great line, but Indian was making more motorcycles than the whole rest of them combined at one point ... !


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