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Random Comment About Blown Spark Plugs

There has been a number of complaints posted about stripped spark plugs and some especially volatile ones about Fords alleged “inferior cylinder head design due to shallow spark plug threads” on some of their aluminum head engines. The same complaint is made on a few other sites about this alleged defect.

Needless to say, I’ve maintained that anyone who has a spark plug thread problem has either inflicted it upon themselves or had it inflicted by either leaving the plugs in place for eternity or overtightening the plugs, which are of the tapered seat variety.

I also maintain that 3 ft. lbs. is sufficient torque for this type of plug and no way on God’s green earth should 10-15 ft. lbs. be used. A bump snug is all it needs.

While doing maintenance on my Lincoln 4.6 (one of the alleged problem children) this past weekend, looking at the plugs got me thinking.

The Mark uses Autolite AP764s and these do have a shallow thread; about 6 turns with the other half of the shank being unthreaded. After 224,000 miles with the plugs being inspected every 15k and replaced every 30k, I’ve had zero problems.

Now here’s my point; along with a few links. If one is going to make the point that the shallow threads are responsible for the plugs blowing out then this should be occurring on a wholesale basis with dozens of other makes of cars fitted with aluminum heads.

Note the plug design of both the Autolite and Champion and then look at the “See all vehicles this plug fits”. And those lists are not even complete as there are a number of other unlisted cars that use the same plug.



I will add a bit of disclaimer on the Champions because I do not believe all of those vehicles use a tapered seat plug and many of them use a gasketed plug.

I agree that these problems often stem from owner or shop caused problems. That said, though, blown spark plugs seem to happen more often on Ford’s engines that other manufacturers. So while the owner may have some responsibility, Ford should, too. You’re right that if it was just the thread design, it would be a widespread problem, but it’s not, so it’s not. Perhaps the aluminum Ford used is too soft?

Sounds like good advice overall. I will suggest that it is a good idea to get the same brand recommend by the manufacturer. I don’t know how true it is today, but in the old days different brands tended to have slightly different thread design and that resulted in some problems. Of course if you forget to do a final tightening on a plug (as I did once) don’t be surprised if you will be replacing the threads in the head.