There is nothing more frustrating that finding out that you have stripped out a set of threads. After installing my "new to me" transmission in my 300zx, I found that lower starter mounting hole had been stripped out. This is rather easy to do; stripping not finding. First the housing is aluminum which strips easily. Second the threads on the starter holes are M10 1.5 pitch. 1.5 is rather course, and most other M10 bolts on the Z are 1.25 pitch. The previous owner must have at one point stripped out the threads and not repaired them.
I was in a bit of a rush when I put thing back together, so instead of fixing it, I locktighted the bolt and snugged it down. The other day when I went to start my car, all I heard was a buzzing sound as the starter pinon gear ground against the flywheel teeth. The lower starter bolt had fell out and the starter was now dangling by the top bolt, barely. Long story short, I make it home and begin to repair the threads.
Helicoils resemble spring coils, wound with stainless steel wire of special cross sectional dimensions, to match the threads they are repairing. The inside of the spring coil matches the repaired thread, the outside matches the next thread size up. Helicoils are often times inserted into a bolt hole from the factory to increase the strength of a threaded hole. An example of this is an aluminum flywheel. To increase the holding strength of the bolts, each hole is helicoiled. The stainless steel wire threads are much stronger than those provided by aluminum ones.
A helicoil repair kit includes several helicoil springs, a tap, and a HC insertion tool, basically a long bolt with a square tip. The first step is to clean out the mangled threads of the existing hole. This is most easily accomplished with a drill and the proper size bit. A drill and tap chart will tell you how big of a hole to drill for the size of the tap. Be careful drilling. Do not ovalize the hole.http://www.kramerusa.com/DrillTapChart.htm
You drill the hole not for the size of your thread, but for the tap, or the outside of the HC. The tap size should be marked on the tap provided in the kit. I had very limited space, so I was not able to clean out the hole with the traditional method. Instead I took the old bolt and a hammer and pounded out the remaining threads. The hole was not exactly the right size, but aluminum cuts easily with a tap.
The next part is to tap the hole. The kit does not include a tap handle, which is very important, so try to get one before you start the job. Otherwise a pair of vice grips sorta work. Starting the tap is the hardest part. Try to make the tap as centered as possible, apply pressure and keep turning until it bites. When cutting threads, turn the tap 2 turns in, one turn out. Use cutting oil, and make sure the hole does not fill with metal chips.
The last part is to insert the helicoil. Thread the HC all the way onto the insertion tool. Apply lock tight to the OUTSIDE of the HC threads. Attach tap handle to the insertion tool. Screw till the HC sits flush with the hole. Let the locktight dry, apply anti seize compound to the bolt and install. There is a little tang at the bottom of the HC that some say should be busted off, but I did not, and do not think it is important. It is important that the bolt does not go deeper than the HC.
Here is another write up with some pictures.
For the top hole there was no way I was going to be able to thread a tap in due to space limitations of the tranny still being in the car. I also found that the threads were mostly good, just the first few were stripped out. So instead of repairing the thread, I inserted a stud, a bolt with thread on both ends. Since the stud was longer than the bolt, the stud make a solid thread engagement with the remaining good threads. Studs also do not come lose as easily. I am not sure why, and have not reasoned out a logical explanation.