How to repair stripped or otherwise damaged threads of a glow plug (g.p.
) without removing the cylinder head
This repair does require that the g.p. is not stuck or broken off in the head. You can see, these threads are totally gone at the top of the hole. Although the cylinder head that the repair was performed on was done on the bench, this process is intended as demonstration for a cylinder head mounted in the vehicle. You may find it necessary to remove glow plug harness, injector lines and vacuum resevoir for decent access. Although the injector in these pictures is removed, that is not required to make this repair.
Insert a piece of 1/4" braided cotton rope or a similar substitute that is about 3" long, into the bottom of the g.p. hole. Completely plug the hole going into the cylinder. I use an awl to push it tightly into the bottom of the g.p. 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 for the g.p. 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.
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 accomodate the insert. I have the kits in stock for $78.
It is 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. You can see the depth marking.
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 g.p. has stripped out the threads, there are still enough threads that are not reached by the threads of the g.p. 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 continure to full depth as marked on the tap.
Reinstert 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 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 iit will blow out. I prefer to try to remove it by hand.
I always treat the threads of the g.p. with anti-sieze.
Also, always insert the g.p. BY HAND and HAND TIGHTEN.
If you use anti-sieze and insertion by hand until the g.p. seats, you wouldn't be rethreading the hole... Of course that doesn't account for whoever got there before you...
and you are done!
Ever stripped an engine mount bolt? Here's how to fix it without removing the engine mounts!
http://forums.tdiclub.com/showthread.php?p=3302166#post3302166
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