Spark Plug Blow Out

Just throwing this out there because, well just because.

Spark plug blow out. WHY WHY WHY?

Spark plug blow outs on the 4.6 engine have happened when new, old, dealer installed plugs owner installed plugs. Why?

  • These half threaded plugs have been used for YEARS, millions and millions of plugs and I don't recall this being such a problem in the past.
  • The half threaded heads? like the plugs they have been used for years .
  • Steel plugs and aluminum heads? Again, millions of these setups and I don't think this is our root cause.

So WHAT makes the later model setups prone to the plug backing out?

Well the only thing I have been able to come up with is the COIL ON PLUG, the COP.

In the old days you had a spark plug wire that basically just hung off the plug, yes there was some type of wire loom but still the plug boot just basically hung on the plug.

BUT we have a COP that is Bolted in place with the plug boot and internal spring constantly putting pressure on the top of the spark plug pushing straight down on the spark plug.


Well my thinking is that as the engine heats and cools, at a different rate than the spark plug, this SLIGHT restriction of plug movement could have a ratcheting effect and unscrew the plug.

As small as this movement might be, if it started, it would continue to get worse.

SOOOOOOOOOOOOOOOOOOO, the next question is WHY does it happen to some and not others?

Seeing as a large majority of people use a torque wrench to install their plugs, probably because of the spark plug blow out we are all more careful to follow the directions, that kind of leaves the amount of torque out of the theory. I believe that the amount of silicone grease in the boot could be a factor. Very simply, adequate silicone grease would allow the spark plug to twist instead of being held in place by the COP Coil Boot. A shortage of silicone grease may allow the boot to restrict the movement of the plug during heating and cooling.

HEY I don't know, it is just something I came up with. BUT I use plenty of silicone grease in the boot and I also use a rubber preservative on the boot while I have it out changing the plugs.

Oh and if you were thinking about leaving the COP bolt out so the COP can move, the bolt completes the ground for the COIL, so don't do that.

No, not Spark Plugs on sale cheap but instead the condition where the Spark Plug Blows Out of the head.

Seems there have been quit a few 4.6 and 5.4 mod motor owners discovering their spark plug has decided to back out. It appears that once the plug is loose then the cylinder compression starts to force the plug up on the compression stroke and then the COP pushes it back down. It does not take long untill the steel plug has removed all the threads from the soft aluminum head.

Ford is quick to determine that it is the owners fault for not putting the plugs in corretly and repairs have been very costly to install a new head. Ford says you can NOT repair the head and it must be replaced. As you can imagine a lot of owners are very upset at this situation especially if they have not changed the spark plugs. I have seen posts in other forums where it happened with vehicles with OEM plugs with 10,000 miles up to 75,000 miles.

You could say that owners are incorrectly torquing down the plugs or over torquing the plug and weakening the threads on the heads. But that does not explain the failures with factory installed plugs.

Your options

1. See if Ford will replace the head under warranty.

2. You can replace the head

3. Or you can repair the head

Clearly door #1 is the best, as long as you don't mind Ford tearing your head off.

#2 is a sure fix but who has the extra money?

Which brings us to #3. In the old days you could have taken the head off and installed what they call a helicoil. Basically you would drill the old threads out and then cut new larger threads into the head. Next you would turn in a new set of steal threads into the over size threads you just cut. This would bring the size back to the original spark plug size.

But there is a new product that from what I can tell, (No I have not had to use it, thankfully) people have been VERY happy with the results. It is a product called Big Sert. Also from what I can tell they are doing the repair with out removing the head.

Basically it is a one piece insert like a bushing. You drill and tap new threads in the head and then screw in a one piece insert. Once the insert is in place the inside of the bottom threads are smaller that the upper threads. This way when you run the supplied driver all the way in you are pushing the sides of the insert into the aluminum head. Locking the insert in to place and forming the last couple of threads.

Here is a link to the company http://www.timesert.com
Here is a link to a step by step repair http://www.timesert.com/bigsert-repair.html

They make a kit just for the Ford 4.6 and 5.4 mod motors. I would call them to make sure you get everything you need. After the repair you remove and replace your spark plugs as you normally would.

Some people have also ordered the complete kit and then had a local repair shop do the repair. This still saved them 100's of dollars.
If any one has a picture of a damaged plug or sparkplug hole could you send it to me to post as an example. Thanks

So what can be done to prevent this from happening to you? Well it is either because the plug was over torqued and started pulling the threads out of the aluminum head or the plugs were not installed tight enough and begin to back out.

Torque for the Mustang GT is 7-15 foot pounds. Or 1/16 th of a turn after the plug is finger tight. I have experimented using anti-seize compound on the threads only, not on the tapered shoulder and set the torque wrench to 13 lbs. With this setup the clicker torque wrench breaks right at 1/16 th of a turn. I have been checking the plugs every 10,000 and so far they have not loosened.

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