Sparkplug thread and Gunson colortune

[Atoll]

I know the spark plug thrread of my A7SS is an imperial tbread but I have been told a 14mm Gunson colortune  will fit. Will it or do I need an adapter? Thanks.

[bsa-bill]

14 mm is the size of the plug, Colour tune  comes with adaptors for various sizes I think


I have a sparkplug chaser for cleaning sparkplug hole threads, never gave much thought to just what thread it is, yep you'd think imperial but it's interesting, motor engines do use a mix of measurements we (UK) use cc for displacement , I think US use inches ??

[Rex]

Spark plug threads have always been metric; 14mm and 18mm being the usual. The Colortune thing will fit, but it won't tell you much on modern fuel.

[KeithA]

I need to buy  chasing tap for 14mm plug. I understand they are not a common 14mm x 1.25 thread but rather a bit smaller. I measured thread dia. of many 14mm plugs and they seem to be between 13.7 & 13.85mm. Not 14mm exactly. Might save a sloppy fit if this is the case.
Cheers

[Greybeard]

A reply from Practical macinist forum:
https://www.practicalmachinist.com/vb/general/ot-why-spark-plugs-metric-293857/

...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.

[Atoll]

Spark plug threads have always been metric; 14mm and 18mm being the usual. The Colortune thing will fit, but it won't tell you much on modern fuel.

Thanks just what I wanted to know. So the 14mm colortune will screw straight in.

[BSA_54A10]

The thread on the colour tune is undersized because it has to work in both parallel threaded plug holes and tapered thread plug holes.
Finger tight is fine the copper sealing ring works really well unless you overtighten them and flatten it.

[Rex]

Certainly will.
To KeithA..just buy a common or garden plug thread-chasing tap. Even our local Halfords used to stock those.
Never seen a tapered thread spark plug. Tapered seat maybe but not tapered thread. What are they used on?

[coater87]

Certainly will.
To KeithA..just buy a common or garden plug thread-chasing tap. Even our local Halfords used to stock those.
Never seen a tapered thread spark plug. Tapered seat maybe but not tapered thread. What are they used on?

 We used to have a hit and miss engine for pumping water that had tapered plugs. I want to say the motor was a Ward, and the plugs were Excell??? That's thinking back 40 years, and that motor was ancient than...

[KeithA]

A 12 mm plug actually measures 11.81mm or .465 of an inch which is about .008 under a standard 12 x 1.25 tap size and a 14 mm plug measures 13.77mm or.542 of an inch and is .009 under a standard 14 x 1.25 tap. I do have several thread repair chasers and they measure exactly the same as the plugs.Jan 13, 2010                                                                                                                                                                                    This above I found on Google. Thought it might be of help to others. My last post, I asked because I needed to chase the plug threads on an A7 iron head. Lucky I didn't just use a standard 14mm x 1.25 tap from my metric set.

[BSA_54A10]

What to use depends upon what damage you are trying to correct.
If it is carbon build up inside the cylinder end of the thread, then a std plug thread chaser would be fine.
If you are trying to repair damage to the top side of the hole . lie a crossed thread then a back tap is the way to go.

[Greybeard]

...a back tap is the way to go.

I had to look that up:
https://www.amazon.co.uk/Powerbuilt-641148-Back-Tap-Thread-Repair/dp/B001SOHXLQ

[BSA_54A10]

If you are thinking about buying one, they are cheaper as a kit of 3.
You can not a 10mm back tap as a single item down here so I had to buy the complete set.
Not needing more than one in any size I sold off the other two on evilbay and they got bit up well over the actual cost price.

As a mower technician I probably have to do about a dozen cross thread repairs a year.