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Old August 3rd, 2008, 04:52   #1
Franko6
 
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Default Pictorial: How to Fix Stripped Glow Plug Threads

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...66#post3302166
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Last edited by Franko6; February 22nd, 2011 at 21:01.
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Old August 3rd, 2008, 05:30   #2
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Very nice write up. If I ever need this info I will have it saved. Hopefully I wont need it.
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Old August 3rd, 2008, 15:28   #3
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This should go into Wingnuts How To Sticky. Very nice. I am abou to change my glow plugs and I bet they will come right out as use anti sieze when I replaced them the first time.
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Old August 3rd, 2008, 16:24   #4
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When I repair these I use red locktight on the insert to prevent it from coming out when the need comes to replace the glow plug again.
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Old August 3rd, 2008, 22:07   #5
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this should be a sticky, anyone could run into this problem, nice write up.
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Old January 29th, 2009, 18:01   #6
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Quote:
Originally Posted by Franko6
.....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.

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.
Does the extended/two stage tap cut any extra threads to start with because of the extra length up front? And if so I assume that's OK and they don't get to the sealing seat further down in there?

I'm going to remove a broken one this weekend, with luck the threads will be fine, but I should probably have that kit on hand in case they don't survive.
I'll have the head off and on the bench or milling machine, and other tips or pointers?

Thanks for the good writeup and pictures!
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Old January 29th, 2009, 19:34   #7
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Great write up I hate to even think of changing mine as they have been in the car since new, 8.6 years. Maybe they will last till I sell it in another 6 years. LOL
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Old January 29th, 2009, 20:06   #8
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The thread piloted tap will cut threads deeper. But there is enough room from the sealing seat.

I was lucky on mine, they all came out easily after 8 years.
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Old January 29th, 2009, 20:11   #9
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Frank, I'm curious how to get the new threads lined up with the old. You said:

Quote:
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.
First, it appears that you aren't drilling the hole to the exact size for the heli coil. I don't tink that htis is a problem because the stripped hole will be very close to the right size and we are talking about Al. so it will cut easier than Steel.

The thing is, the tap will be cutting threads bigger than the threads that are in there, so how do you ensure they are lined up? Or am I overthinking this still? I guess the big question is how much of the GP threads are engaged in the hole originally?
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Old January 30th, 2009, 10:01   #10
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Quote:
Originally Posted by whitedog
Frank, I'm curious how to get the new threads lined up with the old. You said:



First, it appears that you aren't drilling the hole to the exact size for the heli coil. I don't tink that htis is a problem because the stripped hole will be very close to the right size and we are talking about Al. so it will cut easier than Steel.

The thing is, the tap will be cutting threads bigger than the threads that are in there, so how do you ensure they are lined up? Or am I overthinking this still? I guess the big question is how much of the GP threads are engaged in the hole originally?
The tap used is piloted by the original threads, you have to clean out the bad threads in order to use this special tap. It has a reamer in between the old thread pilot and the section for the insert. The pitch does not change so this system works well.

Yes, it will cut threads deeper but there is about 0.80" of clearance between the end of the threads and the sealing cone on the GP.
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Old January 30th, 2009, 10:20   #11
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I see it now. Clear at the (Left) side of the picture are the pilot threads. That makes sense.
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Last edited by whitedog; February 2nd, 2009 at 16:30.
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Old January 30th, 2009, 10:31   #12
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Quote:
Originally Posted by whitedog


I see it now. Clear at the right side of the picture are the pilot threads. That makes sense.
You get the idea...'cept the pilot is at the left. The black marker is showing the depth needed but if you have a plug tap for the insert, all you need is about 3 full threads, then use the plug tap to get full depth.
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Old January 30th, 2009, 10:44   #13
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Quote:
Originally Posted by Keith_J
You get the idea...'cept the pilot is at the left. The black marker is showing the depth needed but if you have a plug tap for the insert, all you need is about 3 full threads, then use the plug tap to get full depth.
Left, right... Yeah. Whatever. Apparently your psychic abilities are subpar and didn't know that I meant left when I said right.

Yes, I saw them on the LEFT, but typed right. My bad.

I was figuring that a plug tap would be the way to go, thanks for the clarification.
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Old January 30th, 2009, 12:36   #14
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There are two reasons g.p. threads are damaged. Cross-threading and over torquing. If you are going to helicoil the hole, the cross-threading is the one that will give you problems. The angle of the shoulder around the g.p. hole is square with the hole. If you can't get a good feel that the tap is chasing the correct path, one technique would be to make an alignment tool that rests squarely against the g.p. shoulder, drill a pilot hole perpendicular through the tool and use that as a guide for your tap.

There is also an expandable rethreader that can be inserted into the threads, tightened up and withdrawn from the hole, chasing the threads backwards. It's perty slick deal...

But seriously, it doesn't matter if you are working with the head on the car or on the bench. The problem is the same...

That is where experience kicks in. As I said in the write up, I will chase the threads with a 10 x 1 tap to make sure I'm going down the right path. If the threads are badly damaged to begin with, I might even drill out the threads to a certain depth and make a pilot hole for the tap. That has to be done with caution, because the drill might get very aggressive and pull into the hole or go off path. As long as you follow the same angle as the original hole, the helicoil will work.

It is a matter of feel. The tap is small and can break. I've seen the nightmares. This whole writeup is an attempt to avoid removing the cylinder head and saving yourself a $1500-2000 repair bill. If you don't have the confidence to straighten out some botched up threading, hand the job to a machinist with the experience. Let him screw it up!

What plug tap? I don't think you understand. The thread size for the helicoil is an oddball size. I don't think you're going to get a tap that size. Just run the two-stage tap in the hole. It will get the job done.
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Old January 30th, 2009, 12:50   #15
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Quote:
Originally Posted by Franko6
What plug tap? I don't think you understand. The thread size for the helicoil is an oddball size. I don't think you're going to get a tap that size. Just run the two-stage tap in the hole. It will get the job done.
My brain is atrophying. Of course there isn't a plug tap for a helicoil unless you make one. I need to find me one of those special taps is what I need to do. I have a back tap, but this would be just the thing. Is it part of the Recoil kit?
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