[Destroyer]

Heli-coils have a special tap that has slightly elongated threads. The heli-coil itself is like a spring...it's not solid. After you thread the hole you screw the heli-coil onto a special inserter tool. When you insert the coil into the newly threaded hole the slight streach of the coil then mimics the regular tpi of the bolt hole. You have to use a special inserter (comes with the kit) to insert the heli-coil, and after it's inserted you have to use a hammer and punch to break off the inserter tang at the bottom of the thread. I've used them on hundreds of different sized holes in my day, but truthfully never did a spark plug hole with them. 5/16x18, 3/8x16 and 1/4x20 are the most common sizes used. They work well on bolts that are not removed on a regular basis.

The problem with heli-coils is that they tend to loosen up or come out with bolts that are removed over and over, so they aren't the best bet for spark plug holes where you remove the spark plug on a normal maintenance basis. For holes like that I highly recommend genuine time-serts (www.timesert.com) because they do several important things. 1) They have a shoulder on the top that provides a positive stop and prevents the insert from threading itself deeper into the hole. and 2) the last few threads of the insert are not fully finished, so that as you screw the insert into the hole the bottom threads expand, and that permanently locks the insert into the head metal.... no locktite is needed. They are a little more expensive, but in my opinion, they are well worth the extra bucks .