I've taken the head of my engine and seen that the spark plug threads are not the best, so I bought a helicoil kit to fix it. The spark plugs are M12 x 1.25 and I bought an M12 x 1.25 kit. When it arrived, the tap is M12 x 1.25 and the coils are 12mm OD. I used the tap to clean up the existing threads.

Have I got it wrong? I thought the quoted size of a helicoil is the inside thread of the coil, not the outside thread.

The ones I have bought were always the finished sizes, ie M8 but they were not genuine Helicoil.

The ones I've used are just the size of the bolt going into them, you should be able to screw it into the helicoil. They probably sent you the wrong ones. A quick google gives the proper drill size for M12 as 31/64" or 12.3mm. There's tons of tables on the web.

Tbh for all the trivial cost I'd just give it to a machine shop to do as they'll have done thousands and a mistake with the head, well it could get expensive. Mind to use copper slip on the spark plug threads to prevent future issues and improve plug cooling (so many people leave that out!).

Just my 2p but I'd be very careful with copperslip on the threads as you can overtigten if not careful, particularly on taper thread plugs

From the NGK Website:

"1. Anti-seize

NGK spark plugs feature trivalent plating. This silver or chrome-colored finish on the threads is designed to provide corrosion resistance against moisture and chemicals. The coating also acts as a release agent during spark plug removal. NGK spark plugs are installed at the factory dry, without lubrication or anti-seize.

Anti-seize can act as a lubricant, altering torque values up to 20 percent, increasing the risk of spark plug thread breakage and/or metal shell stretch. Thread breakage can sometimes involve removing the cylinder head for repair. Metal shell stretch changes the heat rating of the spark plug and can result in serious engine damage caused by pre-ignition. Do not use anti-seize or lubricant on NGK spark plugs. It is completely unnecessary and can be detrimental."

I suspect you have maybe a kit for 10x1.25 threads ?

quote:

---

Originally posted by rusty nuts
I suspect you have maybe a kit for 10x1.25 threads ?

---

quote:

---

Originally posted by perksy
Just my 2p but I'd be very careful with copperslip on the threads as you can overtigten if not careful, particularly on taper thread plugs

From the NGK Website:

"1. Anti-seize

NGK spark plugs feature trivalent plating. This silver or chrome-colored finish on the threads is designed to provide corrosion resistance against moisture and chemicals. The coating also acts as a release agent during spark plug removal. NGK spark plugs are installed at the factory dry, without lubrication or anti-seize.

Anti-seize can act as a lubricant, altering torque values up to 20 percent, increasing the risk of spark plug thread breakage and/or metal shell stretch. Thread breakage can sometimes involve removing the cylinder head for repair. Metal shell stretch changes the heat rating of the spark plug and can result in serious engine damage caused by pre-ignition. Do not use anti-seize or lubricant on NGK spark plugs. It is completely unnecessary and can be detrimental."

---

quote:

---

Originally posted by perksy
Just my 2p but I'd be very careful with copperslip on the threads as you can overtigten if not careful, particularly on taper thread plugs

From the NGK Website:

"1. Anti-seize

NGK spark plugs feature trivalent plating. This silver or chrome-colored finish on the threads is designed to provide corrosion resistance against moisture and chemicals. The coating also acts as a release agent during spark plug removal. NGK spark plugs are installed at the factory dry, without lubrication or anti-seize.

Anti-seize can act as a lubricant, altering torque values up to 20 percent, increasing the risk of spark plug thread breakage and/or metal shell stretch. Thread breakage can sometimes involve removing the cylinder head for repair. Metal shell stretch changes the heat rating of the spark plug and can result in serious engine damage caused by pre-ignition. Do not use anti-seize or lubricant on NGK spark plugs. It is completely unnecessary and can be detrimental."

---