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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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Yes, the thread pitch is standard on spark plugs - 1.25 on 12 and 14mm plugs, 1.0 on 10mm plugs and 1.5 on 16mm plugs
There's a reason the kit you found on Ebay is cheap - it's a cheap fix that's not very good quality wise. I've seen those inserts come out with the spark plug.
A better fix is a Helicoil thread insert - more permanent and correctly installed will never come out unless the spark is grossly over tightened. The Helicoil provides a stronger thread than the original hole, but unless you want to take the head off, it's best to take it to an experienced mechanic to have it done right. In my shop, I set the engine about 90 degrees before TDC of the exhaust stroke with the exhaust valve open - I then connect the shop compressed air supply to the exhaust outlet of the muffler - with shop air flowing through the engine and out the spark plug hole, I use a special combination piloted reamer tap to cut the hole to an oversize and thread it for the insert in a single pass. The airflow blows the chips from the reaming and threading out of the cylinder via the flutes in the tap. That was the easy part. Now you use a special tool to pre-wind the Helicoil to insert it into the spark plug hole. When the Helicoil is nearly all the way in, you remove the pre-winder and set the Helicoil about 1/2 thread below the surface of the head. Now the hard part; there's a tang that the insertion tool hooks on to screw the Helicoil insert into the hole that has to be broken off and removed from the cylinder. You have to ensure the tang is removed or engine damage can result. If the head is removed, then it's a simpler job to remove the tang.
You can attempt the repair yourself, or bite the bullet and have it repaired by an experienced mechanic. It might be cheaper initially to repair it yourself, but I might be cheaper in the long run to have it professionally repaired. It's your machine and your call, but a cheap fix is usually not good and a good fix is not cheap.