Spark Plug Thread

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Jun 12, 2007
I've just taken the head off and decided to get the sprak plug threads redone while it's off. They were getting a bit stiff and thought it was just a matter of time.

So what size thread should go on? I'm told it's a 14mmx1.25 but the guy just rang me up to say the whole is more like a 12m???!! Is 14x1.25 right. I was going to use champion N3s as that's what Mick Hemmings recommends?

As always, thanks very much
14 X 1.25 is correct, has someone put inserts in to take 12mm plugs, if so why, a cheap job lot.
I can't see why. I would have thought 14x1.25 was what it was already but the guys in the shop may just be being over cautious. They said it was a poor measurement.

There is some misunderstanding about metric pitch that holds on even with the experts. A pitch of 1.25 means that if you were to turn that thread exactly one full turn 360 degrees it would change in position 1.25 MM .
It is not like other thread pitch call outs as in so many threads per inch. You would be very surprised at how many Machinist's do not understand this and still cut the threads. If it were me asking for the repair I would take in the spark plug and tell them to fit it up. Gees
Well that was of course my first idea but I'm in a different part of the country to my spark plugs and it 'd take a few days. Thanks to all.
Why not buy another spark plug and take it or mail it to the machine shop?

BTW, if you ever have to have your exhaust threads redone (and you will, sooner or later) be sure to send the exhaust nuts you want to use along with the head. Ask me how I know...

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