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Spark Plug Insert

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Giles

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#1
Seems like I run into a lot of stripped or damaged spark plug threads, especially with aluminum heads.
A few years ago, I was fed up with failed heli-coil and inserts, so I decided to make my own.
The tricky part is tapping the 3/4-16 thread to correct depth to allow a correct thread interference.
After accomplishing this, I apply a permanent high heat thread locker to insert and installed INSERT.JPG Insert 2.JPG INSERT3.JPG insert4.JPG with a spark plug.
Been doing this for years with never a problem.
 

RandyM

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#2
Nothing like making your own quality. :encourage: It is hard to buy it today, either it is not made at all, or it is way expensive. Kinda satisfying yeah?
 

Giles

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Nothing like making your own quality. :encourage: It is hard to buy it today, either it is not made at all, or it is way expensive. Kinda satisfying yeah?
Yeah--the wonders of a Lathe--The master Machine.
 

machPete99

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#4
I believe I saw something similar on a Kohler engine, installed from inside the combustion chamber, probably done at the factory, on aluminum K series heads.

Is that how yours is to be installed or is it from outside?
 

Giles

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#5
I believe I saw something similar on a Kohler engine, installed from inside the combustion chamber, probably done at the factory, on aluminum K series heads.

Is that how yours is to be installed or is it from outside?
I have always installed from outside. With some heads, spark plug is at an angle and would be very difficult to install from inside.
If you noticed the flange on the insert, Head is counter bored 020.
That insures a bottomed out assembly with flat sealing surface.
Had a guy that was skeptical and said it wouldn't work.
That was three years ago and he says it's still holding.
 

Giles

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#6
Something I have never understood about spark plug threads.---
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 just use the standard size tap and have had no problems.
I have never seen a special size spark plug tap available.
 

Guv

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#7
The secret is that you make the insert the right length to match the spark plug . I have seen many insert turn out with the plug because they are too short and carbon builds up on the spark plug thread that protrudes the insert. I have fitted many inserts without any failures. There is also a special insert with pins in the flange part that after you tapped and screwed the insert into the head you knock the protruding pins into the aluminum cyl head to prevent it from coming out when changing spark plugs.
 

Giles

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#8
The secret is that you make the insert the right length to match the spark plug . I have seen many insert turn out with the plug because they are too short and carbon builds up on the spark plug thread that protrudes the insert. I have fitted many inserts without any failures. There is also a special insert with pins in the flange part that after you tapped and screwed the insert into the head you knock the protruding pins into the aluminum cyl head to prevent it from coming out when changing spark plugs.
Sir--You are correct and I agree. I have never tried to remove one of my inserts but I would suspect it would be difficult, considering the tightness of thread fit, flange, and high heat resistant thread lock that would have to be heated much hotter the cylinder head ever gets.
With my inserts, there are no exposed threads. If there was, that can cause pre-ignition.
 

KBeitz

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#9
Quote... I have never seen a special size spark plug tap available.

They are made. I have one...
 
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