Stripped glow plug repair
How to repair stripped glow plug thread holes
This article shows how to DIY repair stripped glow plug threads in your VW TDI engine or Audi TDI engine without removing the cylinder head.
The article is mirrored with permission from the original author Franko6. He is a TDI cylinder head specialist so if you need any work you can contact him through his forum profile. He also sells the taps. The cylinder head repair is shown off the engine for demonstration but can be used on heads still on the engine. You may have to remove the glow plug harness, fuel injector lines, or other parts for good access. The fuel injectors were removed but are not required for this repair.
Glow plugs threads can be damaged from overtorquing or cross threading. The cylinder head is aluminum and the glow plugs are steel. The repair procedure assumes that the plug isn't broken off in the aluminum cylinder head. If they are snapped off you could try welding a nut to it and using that to torque it out. If that fails you could continue welding nuts or a rod to it.
As a general tip, it should be easier to remove tight glow plugs on a luke-warm engine and use a penetrating lubricant like PB Blaster to soak the threads. The glow plugs can break around 26 ft-lbs which isn't a lot so take your time when removing them. The torque spec for most TDI engine glow plugs is only 11 ft-lbs which is fairly light so if you don't have a torque wrench, get one.
Procedure to retap and repair the glow plug threads
Insert a 3" long piece of 1/4" braided cotton rope or a similar substitute into the bottom of the glow plug hole. Completely plug the hole going into the cylinder so that metal shavings don't fall into the engine. I use an awl to push it tightly into the bottom of the glow plug hole.
Next, take a small piece of cotton and cover over the top of the cotton rope. This is just a extra margin of protection to keep any aluminum or debris from going into the engine. I prefer to chase the threads with a 10 x 1 tap but you may skip this step if you feel confident that the threads will not cross up on you any further. Maintaining the same line as the original threads is crucial. If the threads are severely damaged you can drill the threads out to a certain depth and make a pilot hole for the tap. I use a Recoil brand 38100 with a two-stage tap. The front of the tap has the 10 x 1 mm chaser with a larger tap thread to accommodate the insert. (contact Franko6 through the forum if you need this kit).
It's important to get the hole threaded to the right depth for the insert you will use. I use a 10mm long insert. Measuring against the special tap, mark the tap for depth. You don't want to go too deep. The depth marking is indicated with black marker.
The insertion tool has an adjustment screw for different length inserts. Adjust to collar of the tool so that the tang of the insert is firmly caught in the bottom of slot in the tool. Please note that the insert is facing the direction as it would be installed; tang first.
Insert the special tap and make sure you are using the original thread path. Even if the glow plug has stripped out the threads, there are still enough threads that are not reached by the threads of the glow plug to keep the tap on track. Just be sure to keep the tap square in the hole. Use an aluminum tapping lubricant to ease the tap and improve thread quality.
The tap will begin to feel tight before reaching full depth. Do not force the tap.
Remove the tap and you will see a pile of chips driven in front of the tap. Remove the chips and continue to full depth as marked on the tap.
Reinsert the special tap and work to depth. Remove the tap as necessary to facilitate working to full depth. In the following picture there is about 1/2 a turn to go.
Remove the tap and blow out all chips from the hole.
Apply a drop of motor oil on the insert. Use the insert tool to install the insert into the new threads. Work slowly and do not drive the insert in any more than flush with the top of the hole. Keep the insert 'closed up'. Pull lightly out on the insert tool as you drive the insert in. If you push the insert in, you can skip a thread and cause real trouble.
When the insert is to sufficient depth, break off the insertion tang. Push straight down with a small screwdriver and it will snap off. I retrieve the insert's tang with an awl and magnet.
Blow out the hole again. Retrieve the cotton and rope plug. If it is too stuck to pick out, you can use the engine's starter to pop it out. First compression stroke and it will blow out. I prefer to try to remove it by hand.
I always treat the threads of the glow plug with anti-seize. Always insert the glow plug by hand and hand tighten! If you use anti-seize and insert it by hand until the glow plug seats, you wouldn't be rethreading the hole. Of course that doesn't account for whoever got there before you.
Torque spec for most TDI engine glow plugs: 11 ft-lb