TDegenhart
Diamond
- Joined
- Mar 26, 2011
- Location
- Geneva Illinois USA
For you history buffs, why are spark plugs metric, when until recently, the rest of the engine was english?
Tom
Tom
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OT - Why are spark plugs metric?
- Thread starter TDegenhart
- Start date
- Replies 49
- Views 22,069
Bill D
Diamond
- Joined
- Apr 1, 2004
- Location
- Modesto, CA USA
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.
Bill D.
History of Spark Plugs - A.S.E.C.C.
jkruger
Stainless
- Joined
- Feb 21, 2012
They are metric, but also very close to 9/16-20. Why? No clue.
johnoder
Diamond
- Joined
- Jul 16, 2004
- Location
- Houston, TX USA
Ford Model T and Model A were not metric
Bill D
Diamond
- Joined
- Apr 1, 2004
- Location
- Modesto, CA USA
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.
Bill D.
RC99
Diamond
- Joined
- Mar 26, 2005
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...
EPAIII
Diamond
- Joined
- Nov 23, 2003
- Location
- Beaumont, TX, USA
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.
TDegenhart
Diamond
- Joined
- Mar 26, 2011
- Location
- Geneva Illinois USA
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
Model T Ford Forum: Head with millumeter spark plugs......
Tom
Mud
Diamond
- Joined
- May 20, 2002
- Location
- South Central PA
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.
Ray Behner
Diamond
- Joined
- Mar 27, 2009
- Location
- Brunswick Oh USA
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
Spark Plugs Photos by rbehner | Photobucket
jhovel
Aluminum
- Joined
- Apr 25, 2011
- Location
- Bendigo, Victoria, Australia
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....
Ford tractors and cars made outside the US have also used only metric spark plugs too....
crossthread
Titanium
- Joined
- Aug 5, 2004
- Location
- Richmond,VA,USA
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.
Jaxian
Stainless
- Joined
- Feb 24, 2013
- Location
- Santa Cruz
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.
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.
Mud
Diamond
- Joined
- May 20, 2002
- Location
- South Central PA
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.
Ray Behner
Diamond
- Joined
- Mar 27, 2009
- Location
- Brunswick Oh USA
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!
James H Clark
Stainless
- Joined
- May 11, 2011
- Location
- southern in.
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
JH
roll-a-leblond
Cast Iron
- Joined
- Jan 18, 2014
- Location
- louisiana
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!
