Had to replace the Helicoil on a cylinder after the insert was torn out attempting to chase the threads with the crap ATS 20200 chaser which is about 10 thousands larger than a new spark plug. Threw the chaser in the trash and ordered two of these in case things didnt work out and it was a good thing! These are sized nearly perfectly for narrow or wide deck cylinders so if your depth stop is set slightly too long you could insert it into the cylinder and need to rip it out and try again. I recommend setting the depth stop a couple of threads shorter than your spark plug, then removing the stop and turning the bare insertion tool while visualizing the stakes so you can position the Helicoil precisely where you want it to stop. Didnt have a very expensive aircraft Helicoil staking tool so used red Loctite when inserting in case plier staking isnt sufficient.

For long reach plugs - use a tool that’s in good condition and make sure the threads are clean and oiled

Evergreen A Verified Purchase

Heli-Coil Insert 18MM X 13/16

Quality product - works as intended. May look oversized when you get it but it has to be to install, fit, and seat properly.

Integrity A Verified Purchase

Heli-Coil Insert 18MM X 13/16

The low reviews on this page come from not knowing how to insert the helicoil. The helicoil looks too big, but with a special tool, it is inserted into the cylinder with a bit of spring tension so that when you pull the tool out it is wedged in there.

Fit perfectly. Understanding how the installation tool works is critical! I used a NAPA Balkamp 18-1.50mm spark plug thread repair kit P/N 770-3066.

Size .5mm off. Material possibly to thick. Could not use. Might work for non-aircraft use.

We ordered an 18 mm helicoil for a Lycoming engine. The Helicoil came in a bag labeled 18 mm but definitely was not 18 mm. it measured more than 20 mm on the ID. We had an additional 2 days down time on this engine while we procured the correct helicoil.