some of the missing parts of this thread retrieved:
"I tried to get the Wurth system but could not get it in a reasonable time and price so I had to settle for second best... I don't think it's gonna be a problem, I've even seen helicoils used on sparkplug threads and I haven't seen one fail yet. But I will use Loctite to bond them in... Just hope I'm not gonna run over my own turbocharger. I forgot to add that some of the threads have already been helicoiled prior to me getting the head and they are the only ones I haven't had problems with..."
"I was not referring to the holding strength Samo - merely the ease of use. I will just point out (before anyone accuses me of saying they are no use) that a Helicoil thread repair will never fail per-se - under normal preload/loading and there are reasons for this, firstly that the thread major diameter of the joint is now some 10% bigger so the thread shear stress is commensurately lower in the parent material. Installing Helicoils makes the joint stronger not weaker, the old Lancias had them in all threads. After significant test and research I introduced an important programme of Helicoil threading on new units at Napier Turbochargers when I worked there as a Chief Engineer, both to enhance joint strength and (because the inserts are stainless steel) to reduce the risk of seizure. BUT! To the point about installing a Helicoil is that the tap tool that cuts the thread for the insert has to follow the old thread, which is the very thing that is damaged and the risk of runout during tapping is as high as the old thread is damaged. This can be a real problem when doing spark plug threads (they may end up off-line) - though for compactness they have no equal in that application. Whereas - with Wurth Timesert the first tool is a drill which removes ALL the old thread so the tap goes into a virgin hole. Of course alignment is important but you cannot drill out the old thread with a Helicoil repair. Much depends on the engaged length and setup height of the Helicoil whereas the Timesert has a flange to set the position and a hand-driven counterbore tool to cut the region for that. Timeserts are aval in different lengths too. I write for the benefit of all readers, capitals are for emphasis (not shouting), I trust you understand my points. The only exception I must make clear is that I have long since ceased to use Timesert for spark plugs of any size as for a time I found them coming out with the sparkplug on removal. Whether the design has now changed (as I urged Wurth UK to do for some years and then gave up) I don't know but don't care frankly. For the reasons stated and as I have said in my book I use Timesert on M6 and M8 predominantly."
"Very interesting! I didn't know that helicolil uses the old thread! The system i am using is in fact a generic German brand that uses an improved approach. In the kit you get a 8,3mm drill to firstly remove any residue of the old thread and this way you start with a vigin hole which you then tap with a M10 x 1,25mm hand tap. I think I will use an old bushing over the hole for tapping to keep the tap vertical. I did a test yesterday and I taped a M8 thread in a 15mm thick aluminium plate and then striped the thread with a bolt and a long bar. It really didn't offer much resistance. Then I coiled the thread and tried again and found it's much stronger. Of course as you wrote if the bolt thread doesn't let go then the coil which is harder surley won't let go and the outer thread is bigger which means it's stronger! TO ALL: After spending another couple of hours polishing the combustion chambers and comparing my pictures to the pictures in Guys book. All I have to say is my hat's off to you sir. Even the most irelevant thing like taking a good ilustrative photo that shows what you wanted to show isn't easy. Let alone all the work. The major work of shaping, deburring etc. isn't that hard with some guide and help but as soon as you think you've got the hang of it your burr slips and skids across the head plane... And if I managed to get the port's looking at least half decent I had to admit defeat with the CC's! It's just not possible for an amateur to achive this kind of results without guidance from stage one...
Very interesting! The major work of shaping, deburring etc. isn't that hard with some guide and help but as soon as you think you've got the hang of it your burr slips and skids across the head plane...
Just a comment on porting and chamber work. Always protect the head deck and manifold faces with old gaskets or tape. Very maddening to do all of the work just to run a groove across the nice machined surface."