For those that might have interest in the inserts I use....
I originally made some on my own probably around 35 years ago -- the first time I worked on an engine that had pulled a head stud on the exhaust side of a KT100 cylinder. My original design (which has never changed) was/is: .600" long, 7/16 x 14 OD thread, and of course M8 x 1.25mm ID thread.
The reason for the the 7/16 thread is three-fold:
1) It keeps the OD of the insert as small as possible/practical, which prevents it from cutting too much into the cast-in-place liner on a KT100,
2) It allows the use of a ⅜" end mill to "tap drill" for the 7/16 x 14. A 4-flute end mill is very stiff compared to a drill, and since the existing hole will at least be partially butchered from being stripped, the end miill will cut much straighter than a drill, and it also will deflect very little if it ends up touching the cast iron.
3) The 3/8" tap drill (or end mill) will *just* clean up a stripped M8 heli-coil. So if the cylinder had been fixed once that way, but the heli-coil didn't last long (not unusual), then the ⅜" diameter tap drill (end mill) will just clean up the major diameter of that stripped heli-coil thread.
Not long after I started making those inserts (on my engine lathe), I had a buddy with a small gang-tooled bar-feed CNC lathe make them for me. I think my first order back then was 1000 inserts. They were single-pointed on both the external and internal threads. I wanted the 7/16 x 14 to be right on the high-limit of pitch diameter (so they would fit quite tight on a standard tapped thread), and also wanted a very good fit for the studs so that was cut just a touch tight. I've never changed that approach.
I've had a web page online for close to 25 years that shows how to install the inserts in the top of the cylinder.
It can be found here:
http://muller.net/tech/howto/insert/index.html
If you have a Bridgeport (or even a drill press), that can be done quite accurately by following those directions.
Over the years, i've installed those inserts in not only the top of the KT100 cylinder and the exhaust stud holes, but also in the base of a variety of engine cases.