Ruined spark plug threads

Aluminum heads warp when overheated. That's what seized your plug.

 

If the threads on the plug extended into the combustion chamber slightly, a little carbon deposit there can rip the threads out when removing it. Measure the "reach" before putting plugs back in, shorter ones are available.

 

Don't use any oil, or anti-seize on spark plugs.

 

I disagree, I always use Copaslip, never had a problem in thirty years, and I've done lots of engines with aluminum heads.

 

Not my experience, plenty of experience with stuck plugs when some sort of lub is used. Maybe Copaslip is the better stuff.

Plenty of experts says don't use it, but what do I know I work on Porsches. But seriously, it would not present a problem for you because you probably don't use a torque wrench since you have been doing it for so many years that you just go by feel.

Here's what Autolite says about using anti-seize (snicked off the net somewhere):

We do not recommend the use of any anti seize products for installing spark plugs. Anti seize compounds are typically composed of metallic, electrically conductive ingredients. If anti seize compounds come in contact with the core nose of the plugs, it can lead to a misfire condition.
Anti seize compounds can also have a torque multiplying effect when installing plugs. This can lead to thread distortion and thread galling resulting in cylinder head damage. (Geeze, sounds suspiciously matches It's Ross's description) Autolite spark plugs are nickel plated to resist the effects of corrosion and seizing. However, plug seizure is aggravated further when steel plugs are installed into aluminum cylinder heads for a long period of time.

Here's what AC/Delco says:

Do not use any type of anti-seize compound on spark plug threads. Doing this will decrease the amount of friction between the threads. The result of the lowered friction is that when the spark plug is torqued to the proper specification, the spark plug is turned too far into the cylinder head. This increases the likelihood of pulling or stripping the threads in the cylinder head. Over-tightening of a spark plug can cause stretching of the spark plug shell and could allow blowby to pass through the gasket seal between the shell and insulator. Over-tightening also results in extremely difficult removal.

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heres the Pelican Parts explanation
" Install each plug into the cylinder heads without using any anti-seize compound. Torque the spark plugs to 25 Nm (18.4 ft-lbs). While writing "How to Rebuild and Modify Porsche 911 Engines", I discovered that Porsche doesn’t recommend the use of anti-seize compound, as detailed in Porsche Technical Bulletin 9102, Group 2, identifier 2870. The bulletin applies retroactively to all Porsche models and the theory is that the anti-seize tends to act as an electrical insulator between the plug and the cylinder head. This could have detrimental effect on the firing of the spark due to the loss of a good, consistent ground connection. Keeping those findings in mind, I would make the same recommendations for the BMW cars."

 

I always use anti-seize on spark plugs in aluminum heads. It is applied carefully and plugs are hand tightened. I have never had a problem with this. It is true that lubricating threads can result in over-torquing if you aren't careful.

Some fasteners are meant to be installed with oiled threads.

 

You're correct in that I do not use a torque wrench for exactly the reasons stated. On the race engines I use solid copper washers so they do not crush, another reason I do it by feel. Never had a plug loosen yet

(The reason for solid copper washers is so I can clock the electrode, I make my own washers to fit each hole.)

I have heard bad things about moly based antisieze, but Copaslip so far has been no problems and I'm careful enough to not smear it where it could conduct a spark.

 

As I suspected, you don't use a torque wrench because you are much more experienced. But most inexperienced DIYers do, and using anti-seize will cause issues for those that uses torque wrenche. so the best thing for 98% of people is not to use it.

 

I've seen spark plugs strip from over tightening but I've never seen anti-seize cause thread galling. That the very thing it is designed to prevent.

 

Yeah, I'll stick (no pun intended) with the anti-seize.

 

Threads do wear over time. It's very possible that some portion of the threads on the head (usually it's a chunk about 180 degrees of the thread circle) came loose simply from repeated plug replacement over the years. Aluminum is a poor material to thread anything into really (compartively to steel), particularly for a finer thread such as what's on a spark plug. As the thread broke free from the head it would drag through the threads on the plug as it's being unscrewed and then likely compact into a local area causing the following threads to potentially cross thread. Sometimes the plug comes out OK and you're left with a missing portion of the threads on the head - this really calls for a helicoil as well, even if the plug came out OK.

With the head out it's really not a big deal putting in a timesert/helicoil. The plug bore is usually not so close to a valve seat that it presents any kind of problem. Just my opinion but I really don't think the warp caused by an overheated head would cause a plug to bind - if you put a very precise straight edge on your typical heat warped head it's a matter of hundreds of an inch across the entire span of the head. The plug hole isn't going to distort enough to take up the tolerances in the usual spark plug bore.

 

The first thing you did wrong was stop, and start screwing it back in. The back and forth ruined any possible chance of removal. Once you screwed it back in, you were screwed (no pun intended). Your best off to just keep winding it out. If its going to sieze or snap off, it would anyway.

I dont know if its a written law or anything, but they (aviation mechanics) all use anti seize on aircraft plugs, and those are out every year or every 100 hours.

If your changing plugs often enough its usually not a big deal. But after 10K miles or a few years, they can start to bond themselves in really good. Heat cycles and environmental conditions, its all working to destroy the things man makes of metals. Leave it alone and nature will eventually reduce our work to rubble.

Stick with anti-seize and less than extended maintainance intervals if you want to make it last and still be able to take it apart.

 

Repair the threads using a "timesert" do not use a helicoil as they are not truly designed to handle the pressure of compression and combustion. A timesert are locked into place durring the install process and have a flange that further spreads out the loads and helps to seal the repair.

http://www.timesert.com/html/sparkplug.html

 

I did this to my old Ducati and I was pissed. I somehow managed to put of my Guzzi plugs (long reach) in the front Duc (short reach) head and didn't even realize it. I guess there is plenty of room in the combustion chamber for the extra plug because it ran fine, I probably didn't pull the plug for 1000 miles. It started to come out fine but then felt bad. And from there it was all bad. If I had realized the plug was long I might have pulled the head and Dremeled off the end of the plug, I might have been able to run a tap through and save it. But probably by the time I felt it was bad I'd wrecked half of the threads anyway.
I pulled the head - thankfully the front head is "easy" - and a machine shop pressed in a Timesert. I think it cost $40. It has been fine since, no issues. And I will never make that mistake again!

 

I too have been using the regular antiseize for thirty plus years on spark plugs never I mean never had problem.

 

What type of vehicle did this happen in?
A Ford product, perhaps?

I've seen a lot of stripped, blown out plugs in the Ford engines due to:

-100K mile plug change intervals
-Tapered seat plugs being used in aluminum heads

A little more info might be helpful....and FWIW, anti-seize didn't cause this problem!

David

 

A truly piss-poor design.

 

I guess this just proves that everyone's experience is a little bit different. I've used a drop or two of motor oil on the threads of spark plugs for over 45 years, in dozens of different types of cars, from old American to new American, Jaguars, Ferraris, MGs, Triumphs, Chevys, Fords, Toyotas, Hondas, you name it. I've never had a problem getting a plug out and never had any other spark plug related problems. YMMV, but I'm going to keep using that drop of oil.

 

The Ford modular V8s are notorious for stripped plugs. But for all brands, the long plug change intervals was driven by the EPA with the 8yr/80k mile emission warranty. So long life plugs were installed to prevent catalytic converters from being ruined by misfire. Even though the steel plugs are plated, over time they weld themselves into female threads.

 

Regardless of speculation of what may or may not have happened the fact still remains you have a stripped plug. The anti-seize is definately the way to go with all aluminum/different alloy applications but I've had failures using it also, (e.g. the oil drain bolt on the GF's 3.2 Intrepid or head's on the E9 and E21 models). Since you have the head off, as mentioned elsewhere in the thread, Helicoils are an easy, inexpensive fix and the way to go. I believe NAPA sells the Helicoil kit for right around $40.00 which include directions, several coils, drill bit and tool to tap and insert the coil. Good luck and Happy Motoring

 

A great idea!

I have an extensive assortment of thread chasers (not to be confused with taps and dies) that I use frequently.

 

I will continue to use anti-seize on my lug nuts.