Has it ever happened to you? The spark plug has broken off and only the thread remains in the hole. Sometimes one can have fortune in misfortune, then when the entire insertion, including the porcelain shaft comes out. In this case the same has happened as with the manufacturing, only in the reverse order. All together, not a certain reason to remove the cylinder head. Unless the tapped hole is difficult to access. Of course, any repair work can be easilier carried out if the head has been removed. Apart from that, a specific heating of the aluminium for the removing of the steel-thread is also possible from the combustion chamber side.
However there is also the possibility of removing the thread using a, hopefully mild edged screw extractor (Easy-Out), or similar tool. Before drilling it out and inserting the correct thread-jack, one must be sure that sufficient material remains, so that later on, the cylinder head doesn't take revenge by developing cracks. Then there are always companies, who offer a complete repair-set including a combined tap-and-die cutting tool.
Indeed, during a repair job of this type, there are a great deal of metal shavings produced which have the habit of gathering in the combustion chamber. They can be kept to minimum by using grease on the cutting tool and plenty of compressed air. Surprisingly there are repairers who consider the shavings in the engine to be not all that critical. The fact is however, that even with the endoscopes offered nowadays (as a computer extra), not all the shavings can be localised because they hide themselves, e.g., on the side, next to the fire-bridge. So, wouldn't it be better to indded, remove the head?
Even the application of the thread-jack is not without problems. It must be a bit shorter than the original thread, but not much. This, not only because of the strain, but also because the sealing surfaces may not, in any way, be compromised. Now, how does one screw in the thread-jack? Either a suitable guide is part of the package, and/or it is fitted with a small flange (mostly at the end), which can be pinched off using the suitable tool, without it falling into the combustion chamber. 08/11