14mm spark plug threads

Jiles

Well-known Member
I am making some threaded inserts-(helical) inserts.
They are made out of steel and have a seating flange, to repair stripped threads.
I have an old 14mm thread chaser but needed a tap to cut new threads.
I bought an Irwin 14mm x 1.25mm plug tap and have discovered it is .013 larger in diameter then a spark plug.
Thread chaser is exactly same diameter as spark plug.
Needless to say, the spark plug has a small amount of slop which would probably be ok since inserts are steel, but would be questionable with an aluminum head.
If a spark plug has a special 14 x 1.25mm, I was not aware.
 
Why not just buy a 14 mm spark plug heli-coil kit you can get them just about every where they come with the special tap and insertion tool.
GB in MN
 
I think your tap is probably OK. Taps are made to exacting specs, I've never got a bad one.

I've sent a link of a thread profile. Notice the external threads don't come to a point. The threads of a tap do come to a point. I think that is where you are getting the .013" difference.

They do make oversize taps for holes that will be plated or painted, but you would have to specify that.

If there is an "H" spec on the tap, it should say "H3" for a standard tap. If it says H5, H7, H11, it is oversize.

Something you might check, if you want to make another bushing, measure the hole before it is tapped. Your drill may be going oversize if it's dull or been poorly sharpened. The hole should be 12.8mm or .504" (1/2").
Thread profile
 
(quoted from post at 04:37:25 01/15/17) I think your tap is probably OK. Taps are made to exacting specs, I've never got a bad one.

I've sent a link of a thread profile. Notice the external threads don't come to a point. The threads of a tap do come to a point. I think that is where you are getting the .013" difference.

They do make oversize taps for holes that will be plated or painted, but you would have to specify that.

If there is an "H" spec on the tap, it should say "H3" for a standard tap. If it says H5, H7, H11, it is oversize.

Something you might check, if you want to make another bushing, measure the hole before it is tapped. Your drill may be going oversize if it's dull or been poorly sharpened. The hole should be 12.8mm or .504" (1/2").
Thread profile
Thanks --I am a retired Tool&Die Maker and my measurements and work are right.
I even considered grinding the outer threads but the minor diameter will probable be oversize also.
I was searching on another forum and run into a topic same as this.
One poster stated that a regular metric tap was slightly large for tapping a spark plug thread, so evidently he had experience.
I work on a lot of aluminum heads and had one that was busted but had a perfect spark plug thread.
Plug fit nice and tight until I ran the new tap in.
It cut threads and the plug now has sloppy threads.
This is a new one for me.
 
It is consensus on several internet forums is that the tap must be a "Spark Plug" tap. Standard ISO are enough different to
be just wrong. Jim
 
The thread profile and base line are different on a spark plug tap compared to a machine thread tap. I have a "special" set of spark plug taps I bought just because of the issue your having. I ruined a head on a V-4 Wisconsin engine 40 years ago by tapping the spark plug hole out with a machine thread tap. The spark plug blew out after that.

I have seen the exact dimensions in one of my machinist hand books. I will try and find it.
 
It's not really surprising the threads would be different. ISO fasteners date back to around 1950, while metric spark plug threads haven't changed since the early twentieth century.
 
There are different grades of threads. Some call for more or less clearance (for lack of a better word) between the internal and external threads.
Heli-Coil repairs are ok (but not preferred) for plugs that use a gasket, but they are not used for the tapered seat plugs. They will leak combustion gases with tapered seats.
 

There are small differences in the pitch diameter, major and minor diameters of a standard metric tap and a metric "spark plug" tap

They range from .006 to .020
 
44326.jpg
 
(quoted from post at 15:43:05 01/15/17)
There are small differences in the pitch diameter, major and minor diameters of a standard metric tap and a metric "spark plug" tap

They range from .006 to .020

So that means "special tap" ?
 

Yes
As stated earlier...the make special "spark plug taps"

Many auto stores have them for around $10

Not sure about your heli-coil situation....they probably have a kit
 

yup...they do

http://www.autozone.com/test-scan-and-specialty-tools/thread-repair/oem-m14-1-25-fix-a-thread-repair-system/557108_0_0/?cmpid=PS:3:1:90
 
(quoted from post at 16:03:30 01/15/17)
yup...they do

http://www.autozone.com/test-scan-and-specialty-tools/thread-repair/oem-m14-1-25-fix-a-thread-repair-system/557108_0_0/?cmpid=PS:3:1:90

Yes--sometimes the work.
As I stated earlier, I have use some that eventually came out.
The ones I am making, have a shoulder for sealing and tightening down.
 

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