Sep 082013

Searching online for Ford Triton blow spark plug issues will immediately turn up a dismaying number of links. I did a little bit of research before buying the V10 and I knew a blown plug was a risk for this engine. Actually a blown plug is a risk for any engine, aluminum head engines in particular, and the pre-2003 (2002 and earlier) Triton engines even more so.

Ford engineers in their not so infinite wisdom decided 4 threads in an aluminum head was more than enough for any spark plug. I wouldn’t be surprised to find that 4 threads are enough if you calculate the thread size, the shear strength of the aluminum threads, the cylinder pressure against the spark plug cross section, etc. Four threads are probably plenty strong enough. I personally don’t believe 4 threads are the cause of the failure, but I do believe 4 threads are what lead to the failure.

Here’s a picture snatched from showing a cross section of the Triton head through the spark plug hole. As you can see, not a lot of threads for the plug to hold onto.


Four thread are probably fine for holding against engine compression, but 4 threads are probably not fine for holding against thread shear when installing plugs. I suspect the blowout problem stems from over torquing the spark plugs during install, at the factory or during plug replacement. It’s pretty easy to see that screwing a steel spark plug into an aluminum hole is going to be at risk of shearing out the aluminum threads.

If you read the stories it seems like the vast majority of spark plug blowouts occur after a plug change. Notice in the previous post I mentioned I bought this truck from a mechanic who told me he had replaced the plugs. I know there are plenty of reports of plug blowouts on new engines that have never had the plugs replaced, but it does seem more likely to happen after a plug change.

This seems consistent to me with thread shear (thread pull out) caused by over torquing during install. Ford must have thought so too because early on after plug blowouts starting being reported Ford issued a service bulletin specifying spark plug torque during install. I understand there is controversy about whether over torquing is an issue or not. The guy at seems to think over torquing is not an issue, that the issue starts with “a plug coming lose for some reason. After the plug becomes a little loose the plug starts to rattle in the treads and works the aluminum until the treads give out.”. I’m not sure I buy that, but hey, pick your poison. I’ve screwed enough steel bolts into aluminum to know that if you’re not careful you can pull the treads out in a heatbeat.

So what do I mean that 4 threads are not a problem, but really are the problem? I think 4 threads probably are fine in terms of holding strength but I also think 4 threads are not near enough to prevent thread shear during install. With more threads you might have more holding power than needed for running, BUT, you also have more thread shear strength to withstand the gorilla manhandling that happens during plug install.

So it’s still up in the air to me whether Ford is to blame, the technicians are to blame, or the shade tree mechanics swapping plugs on a Sunday afternoon are to blame. In any event here I am, the proud owner of a V10 with a blow out plug.

If you want to read more about plug blow out on the Triton heads here are a few long winded threads that should consume a few hours, or more, of your life.

F150online – ford spark plug blow out problem
F150online – Blown Spark Plugs, Stripped Cylinder Head Threads
F150online – Spark Plug Blowout in 5.4 Triton Engine

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