tap drill vs tap size again

Well, the problem with some "rules of thumb" is that they usually don't apply everywhere and one has to be careful with their use. Ideally, the depth of thread engagement should be such that the shear strength of the internal thread is equal to the tensile strength of external thread. The length of engagement to accomplish this can range from less than one nominal diameter to several times the nominal diameter. It all depends on the material strengths of the two different parts, and to a certain extent the thread pitch. That's why the standard height of a gr 8 steel nut is about 1d (for use with a gr8 bolt). If one were to use a brass nut on a gr 8 bolt, the normal thread engagement would not be sufficient to develop the full strength of the bolt and the nut would strip (shear) well before the bolt would break. The thread pitch effect is also the reason why fine threads are stronger than coarse threads, other things being equal.

Mark has a good point with the different standard lengths of Helicoils. For steel on steel (similar strength), the 1d should normally be sufficient. For something like a common steel bolt in cast iron the 1.5d would be more appropriate and for stuff like a common steel bolt in aluminum, the 2d would probably be the better choice.

Rod

• adapter

Oh good lord no. I don't think you need another head. Those thread engagement "rules" are in the interest of achieving the maximum full strength of the external/internal thread combination. That would be the ideal but it is not always necessary. As long as there is sufficient strength in both internal and external threads for the application, then all is well. Normal sparkplug torques are not that great to be near breaking the sparkplug - it is also not a complete cross section like a bolt so it is a lot weaker in tension than a steel bolt would be of the same thread size. For that reason the engagements can be less than would otherwise be called for. I have a HeliCoil sparkplug kit (14mm) that has several different length (called "reach" in sparkplug terminology) inserts. The shortest insert has a reach indicated as 3/8" but only has about 3 or 4 useful threads. At 1.25mm pitch that works out to be only 0.36d. The plugs in my AC-B (14mm) don't engage more than 4 threads. You'll find the longer reach plugs are used in aluminum heads.

As long as you have a remaining head thickness in that area sufficient to get reasonably close to full engagement with the available plug threads, I wouldn't worry about it at all. Even if you can't, you are not blowing out plugs now. Therefore any improvement in getting new threads in place, even if it results in less of an engagement than original, should be satisfactory. If the existing threads in the head are seriously damaged, you ought to try and get an insert installed. It can be done in place if lots of care, time and grease is used to keep chips out of the cylinder. I am curious as to where you are getting leakage though. I take it it is a standard flat seat with a plug gasket and not a tapered seat. Is it because you can't really tighten the plug sufficiently (due to partially stripped or damaged threads) to make the gasket seal properly? I am assuming that the machine work was done correctly and fully perpendicular to the existing thread hole. That's a thing you will need to be careful of when installing an insert - keeping the thread perpendicular to the flat sealing surface so the plug gasket seats squarely.

18mm inserts may be hard to find. I am not sure if they are available from HeliCoil.

Rod

Heli-Coil and the other STI makers make removal tools, the most common looks something like a V-shaped screwdriver blade with chisel edges on the side of the V. The tool is tapped into the insert so that the chisel edges cut into the wire and the insert is simply unscrewed.

For larger sizes, the wire of the insert is simply bent to create sort of a "removal tang" that can be grabbed with a pair of pliers. It's very important that the insert be unscrewed, not simply pulled straight out, because pulling the insert straight out will usually destroy the thread in the parent material that the replacement insert would screw into.

There is also a VERY SLICK variant of the STI, the tangless STI. These were developed by Kato of Japan and sold in the US by Fairchild Fastener under the Coil-Thread brand, although I believe that the Kato patent has run out and other makers make the tangless inserts now.

Instead of a conventional drive tang, the tangless insert that a small notch at both ends of the wire. A retractable tooth on the insertion tool snags the leading notch and the insert screws right into its hole; turning the drive arbor backwards causes the tooth to pivot out of the notch and the tool unthreads from inside the insert. There is a conceptually-similar removal tool that threads into the to-be-removed insert and its tooth grabs the outermost notch so that the insert can be removed.

The ONLY drawback to the tangless inserts is the price of the tools . . . a couple of years ago the 1/4-28 removal tool was close to US $200.

Well, maybe there is a second drawback. Once you've used the tangless inserts you don't ever want to use the tang-type STIs again.

As for the other types of inserts, the "pin" type is a "key-locking" insert (commonly spoken of by the tradename Keen-Sert). Key-locking inserts are also easy to remove, but only by drilling away a part of the insert so that the "keys" can be bent and disengaged from the parent material.

The knurled-top-and-ring insert may have a generic name, but I've never heard them referred to as anything other than "Rosans", which is a plural of the tradename Rosan.

When it comes to replacing spark plug threads, probably the very best insert is the Timesert. Timesert has a website (maybe timesert.com, I've forgotten), and a Google search will turn it up. The special tooling is pricy, making Timeserts impractical for the casual user. Pity.

John