For all you DIYers who have diesels taking thread size 12mm x 1.250 glow plugs that need the glow plug hole repaired or restructured:
I discovered a kit from Time Sert, #4212, that works. It's normally a spark plug repair kit, but it works. It uses a double step die, and I used a 17mm insert. The insert has a flared edge so you must use a 9/16" drill bit to just cut an edge on the newly tapped hole and then use the tap once again to just smooth off the flared hole. Then, oil the insert, put it on the insert tool and screw in all the way. Also use in model W210.

Just remember to vacuum out all the metal chips from tapping and drilling. Use grease on the tap to catch most of the metal chips.

There is no other way of repairing these threads short of pulling the head and welding the hole then re threading it.

Prior to 1995 all MB diesels used a 10mm sized glowplug. There are repair kits for them but not for 12mm.