Spark plug thread

[MichaelJ]

I am looking to find out the difference between spark plug threads and regular threads, Mostly the M14x1.25

[crazy^millman]

Hey where is the guy from the racing company at. I think they use to be NPT threads and then I think they went to NPS threads. Again where is the Guy who works for a racing engine builder.

[Flycut]

As far as I'm concerned the only difference is that spark plug threads have a finer thread pitch then standard threads.

[MichaelJ]

It seems like the spark plug threads have a taper to them like a NPT. It will thread on two threads on a straight cut M14x1.25 and locks up.

 

The M14x1.25 is a finer pitch cause 2.0 is course and 1.5 is fine.

[Camdude®]

is this a thread about a thread?

[Storkman]

Try to contact this guy

 

He works for Rousch/Yates racing

[Kevangel]

I don't think there is a taper to sparkplugs, think they shut-off on angle after threads, on motorcycles they use to have a round steel gasket..been awhile, BUT have a new autolite plug here at shop for truck and miked it with 50 mill mikes and no taper on it. Maybe your cyclinder head threads are damaged? or crud? use to be able to heli-coil them I think, if a tap did not clean them up.

[MichaelJ]

It does fit a motorcycle. Sparkplug threads in just fine into two different heads. But wont thread onto a threaded piece of round stock.

[Flycut]

what did you thread the round stock with?

are you sure you didn't use 14 x 1.5mm (standard thread)?

[MichaelJ]

I threaded the round stock with a carbon steel die. Positive I didnt use the 1.5mm pitch because I matched it up with the pitch gauge. Used the matching tap and took a little material out of the head, which made the spark plug slightly loose(nothing major)and it threaded on just fine.

[Kevangel]

If its a motorcycle...Use to be don't over tighten on warm (hot)head (aluminum) or it may seize up when cold...that is one of the things that lead to heli-coils, head would galled to plug.

[Moldmaker74]

quote:

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made the spark plug slightly loose

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Probably not a good thing. You should use a thread restoration tap, not a thread cutting tap. Your class of fit is going to be loose and you'll risk poping out a spark-plug.

[MichaelJ]

I realize it didnt make it an "ideal" fit, but very little change. You can acutally start the engine with it being only finger tight, I really doubt it will pop out. Thats why I am trying to figure out the thread differences.

 

Thanks for the help.

[ducati]

14 X 1.25 MM is a common metric spark plug size.

12.647 is a pretty close size for the tap drill.

All of the one I have fooled with are straight threads with a gasket seal for the comression.

They may make some that have the tapered fit simlar to automotive for the gasket seat.. but..

I have not seen any that use tapered threads.

I have welded and remachined quite a few but to my limited knowledge.. none have tapered threads used in MC combustion engines..

[MichaelJ]

Thanks for the info guys. I contacted a local who makes aftermarket heads and he told me that he uses a straight bore with NO taper when he cuts them. So I really see nothing wrong with the way I did it. But was confused when my tap cut out material in the head that was already threaded.

[cncfrank]

There are special taps made for racing heads. The M14 x 1.25 is one of them. The last place I worked used them on the chevy nascar heads. I'm not sure what is different about them but they are not standard M14 x 1.25 taps. We called them spark plug taps. Wish I had more information for you.