Anyway, he labelled it my 'secret technique' and asked two questions, 1) How do you accomplish such a miracle, O Wizard?, and 2) Why won't it work with Timeserts?
Since this ain't exactly a Knights Templar hand-me-down to the Masons, I'm happy to oblige. I learned it in the course of a two-year program at the taxpayer-and-industry funded L.A. College O' Motersickle Knowledge. Good thing too, as I have successfully evaded learning very much since the bike makers took the cams from down around yer ankles where they belong and stuck 'em up under the gas tank.
Hereinafter followeth the lesson.
But first (there's always a "but first"), since five minutes of prevention is worth an hour of cure, a few things that may keep you from having to do any of this. I'm talkin' about spark plugs here, but the following generally apply to all threaded fasteners.
Check the threads. A spark plug should screw all the way in with your fingers. You cannot strip threads with your fingers (If you can, call me, I got some work in D.C. for you involving unscrewing several examples of a different kind of poorly functioning 'heads').
If you encounter resistance, STOP. Figure it out and fix it. Sometimes there's carbon or a burr on the internal threads or they're starting to strip. Run a greased spark plug tap through it. Gently.
Some spark plugs have plating on the threads or a burr that will cause them to stick. Ease them with a thread file.
Never, ever, start a spark plug into its hole with anything other than your fingers. If access is a problem, use whatever tools you need, just make sure the motive power is your fingers only. I usually use a rubber plug cap. Starting a plug with a 3/4" air wrench is a sure-fire way to find out if this instructional works.
Use a little anti-seize on the plug threads, especially with aluminum heads, and torque them to 11-18lbsft, or for you godless commies, 14.9-24.4 Nm/Kgm, or as those of us with Royal Enfield manuals know, tighten plug very nice sufficiently to prevent gas passage. I think Mrs. G tried that on me once when I was asleep...
OK, here we go. These steps apply only to aluminum cylinder heads. If you have stripped threads in an iron head, yer screwed. I suggest a quarter-stick of dynamite and an 18mm tractor/flathead H-D plug, wedged into the hole with an anvil, screw jack, or some such fastener.
The only prerequisites for this procedure are physical access and an unobstructed straight shot to the plug hole.
First step is to thoroughly internally lubricate the object you wish to
1) Position the piston far enough down the cylinder that the tap won't run into it, with the exhaust valve open and the inlet valve closed. Remember, the sequence is 'exhaust valve closes, inlet valve opens' so run the piston up 'til overlap starts and back it off a little. That should also ensure the exhaust valve is not open far enough for the tap to hit and bend it.
2) Cram a wad of tiss-yoo paper down through the plug hole so it rests on the piston. Stick yer oil can into the hole and oil the crap out of it (applies to new as well as used tiss-yoo). This is so any chips that escape the next step will be captured.
3) Grease the flutes of the tap. Use a LOT of grease. The chips from the cutting operation will be trapped in the grease.
4) Tap the new oversize threads. Since you should back off a tap every thread-and-a-half or so anyway, use this step to clean the chips off and re-grease the tap.
5) Clean the new threads. Make sure no chips or grease remain.
6) Install the Heli-coil. Make sure the last thread on the insert goes all the way under the lip of the outermost thread in the head or it may pull out later when you least need it. Use a drop of Loctite if you wish.
7) Use needle-nose pliers to break off the installation tang. Grip them tightly and work the tang up and down. If it falls into the engine, you better hope it lands on the tiss-yoo since it's non-magnetic and maybe too heavy to blow out in the next step. Put grease on the flat tip of a magnet anyway and fish for it. Get it out.
8) Remove the tiss-yoo (those medical-type roach clips work good for this) and blow compressed air into the plug hole. This will blow any chips that escaped the safeguards out the exhaust valve and into the drywall in line with the exhaust pipe, where they may be examined at leisure.
9) Install the spark plug and all the other stuff you had to take off to get at it and go riding.
A note to you pros: be sure to charge the customer for 'R&R Cylinder Head' in addition to the insert job. I'm kidding, of course, but it's been done.
I think a word is in order about removing Heli-coils. Why would you want to do that?, you may well ask. Many reasons, but just as an example, you followed all the above steps and are congratulating yourself on the fine job you did and all the time you saved when you notice you just installed a 3/4"-reach insert in a 1/2"-reach hole. If you don't notice this right away, fear not, you will, either when the piston slams into it and makes the big-end bearings go away, or when the piston gets a hole burned through it from pre-ignition caused by 1/4" of red-hot metal in the combustion chamber. Your otherwise lovely shoes will not look so good when you peer into a spark plug hole and see them underneath the vehicle.
You can probably wiggle enough of it loose to grab it with pliers and give it a yank, but you might undo all the good you have just done by yanking the cylinder head threads out with it. Go ahead and try it. I'm kidding about the yanking. It works if you're the least bit cautious and sort of unwind it, and most likely you'll just end up with a six-foot-long insert you can dazzle yer friends with.
If that doesn't work, use a small triangular file to cut a notch down either side of the insert, stick something suitable in the notches and simply unscrew it. This works good on case screw holes as well. Duck soup, he says from the safety of his keyboard.
Now for Timeserts. You can probably do the same thing, but the more solid construction of the Timesert requires the removal of a lot more metal out of the plug hole. There are a lot more chips, and that's the drawback to me.
On the theory that it's always best to start in the cheap seats, the Heli-coil is the first repair attempt to use on stripped spark plug holes. The Timesert is called for when the hole is too wambled-out for the Heli-coil to accomplish a gas-tight seal.
Thus endeth the lesson. I hope it helps. No extra charge for the hyperbole or attempts at humor.