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    Default OT - Why are spark plugs metric?

    For you history buffs, why are spark plugs metric, when until recently, the rest of the engine was english?

    Tom

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    I believe the modern spark plug was invented in France. Before that they had hot plugs. Same reason that ball bearings are metric.
    Bill D.

    History of Spark Plugs - A.S.E.C.C.

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    They are metric, but also very close to 9/16-20. Why? No clue.

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    Ford Model T and Model A were not metric

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    My Gravely tractor uses the old style larger plugs. It was designed in 1918, but it is a metric plug. I think Ford used a tapered pipe thread with no sealing washer. Why does the rest of the world use British pipe thread instead of US pipe thread? Several months ago I asked here and found out the entire world uses 1/4, 3/8 or 1/2 " square drive for sockets.
    Bill D.

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    Quote Originally Posted by Bill D View Post
    Why does the rest of the world use British pipe thread instead of US pipe thread?
    Probably a lot to do with there was never a metric pipe standard and Britain conquered much more of the world when it mattered... US only became powerful on the world stage after WW2 ended simply because the rest of the world laid in ruins...

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    Well, that kinda explains it, doesn't it? If they were invented in France, definitely a metric country (the French introduced the metric system in 1799), and Mr. Champion came from France, then it would seem natural that he would make metric spark plugs. And engine makers would use the metric spark plugs that were available. The rest is just inertia or resistance to change.


    Quote Originally Posted by Bill D View Post
    I believe the modern spark plug was invented in France. Before that they had hot plugs. Same reason that ball bearings are metric.
    Bill D.

    History of Spark Plugs - A.S.E.C.C.

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    According to this posting, metric plugs were not that common and apparently for export only.

    Model T Ford Forum: Head with millumeter spark plugs......

    Tom

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    Quote Originally Posted by Bill D View Post
    My Gravely tractor uses the old style larger plugs. It was designed in 1918, but it is a metric plug. I think Ford used a tapered pipe thread with no sealing washer. Why does the rest of the world use British pipe thread instead of US pipe thread? Several months ago I asked here and found out the entire world uses 1/4, 3/8 or 1/2 " square drive for sockets.
    Bill D.
    Ford used tapered pipe up through 1927. 1928 through 1932 was 7/8" straight thread, 1933 began use of 18mm.
    Quite a few early 20th century engines used tapered pipe threads on spark plugs, not just Ford.

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    Here's a few spark plugs. All early vintage and all ½" or ¾" pipe and 7/8 thread.
    Spark Plugs Photos by rbehner | Photobucket

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    Are you talking abou the US or the world? The rest of the world has never used American threaded spark plugs - only metric.
    Ford tractors and cars made outside the US have also used only metric spark plugs too....

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    Because imperial is gayer then aids?

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    I think it has something to do with how many meters a spark plug will fly when it is blown out of a Ford Triton V8 engine head.

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    OK spark plugs......

    What about tire rim diameter?
    Is that always in inches?

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    Quote Originally Posted by crossthread View Post
    I think it has something to do with how many meters a spark plug will fly when it is blown out of a Ford Triton V8 engine head.
    My brother just spent a fortune on his V10 E350 van getting a helicoil put in the head when it launched a plug out while he was just driving around town. I thought he was kidding when the service guy said this was not uncommon and even more likely to happen on the V8's. Then I checked the internet and was surprised at how often it happens. Sadly he lives too far away for me to just do it for him, then again he's a stock broker so not like he doesn't have the money to pay to fix it so I don't feel that bad.

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    I was under the impression that the more common size plugs historically found that the metric thread versions to be superior to withstand the compression...metric threads were not too fine and not too coarse for the purpose, but the right compromise between the two.

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    Quote Originally Posted by 3t3d View Post
    OK spark plugs......

    What about tire rim diameter?
    Is that always in inches?
    I don't know all the details, but I can say not always. Some '90s Jags that I'm familiar with use 390 mm rims and are really hard to get tires for.

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    And then there were the 10 mm plugs for the Chevy and I "think" Packard? Whose brainstorm was that! After a year's use, they'd snap off. Hell, you could break them like a twig!

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    And while we are talking about spark plugs, Ford introduced 18 mm(I think) in the late 50's.Do they still use the tapered seat and the larger dia. thread? Haven't owned a Ford in a long time.

    JH

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    Quote Originally Posted by James H Clark View Post
    And while we are talking about spark plugs, Ford introduced 18 mm(I think) in the late 50's.Do they still use the tapered seat and the larger dia. thread? Haven't owned a Ford in a long time.

    JH
    Not since the 80s ish. They went to 14mm on modulars, probably smaller now.

    Nissan is using a 12 mm plug I think? It's tiny!

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