2002 w/5.4 eng. blowing spark plugs

Peter, I just had that happen today on my 2000 F150 w/5.4 eng. First time and I hope the last. I am not very mechanically inclined so I limped to the dealership. They are replacing the head.....a 3 day job to the tune of $1700. Luckily I have the extended warranty. I can sympathize. My question is why does this to happen? What causes a plug to blown out of the engine?

Mark

coil!!!).....same

This problem is not new: Old Volkswagen air-cooled flat four cylinder engines where famous for doing the same thing. Somehow the plug gets slightly loose (maybe not properly installed at the factory, inadequate crushable washer/interference fit taper combined with thermal cycling of the head base metal or not enough thread depth machined in the head). When the plug is not properly torqued, there is always a slight amount of radial play - otherwise you'd never be able to unscrew the plug out w/o galling the threads. As the cylinder fires, the pressure changes wiggle the plug in the head threads. This action wears the threads away since they are composed of the aluminum of the head casting. Eventually the plug is expelled and the coil becomes unloaded, burning it out.

"Nukie Poo

My father replaced many spark plugs in VW's back in the 60's, and his impression of this problem was a slight difference in thread shape between American spark plugs and foreign spark plugs. You could tell the American plugs would fit a bit looser than the German and Japanese plugs, and the American ones were always the ones blowing out. Put some NGK's or Bosch plugs in the VW's, and the problem would not occur.

Gerard

heads......

Actually, now that you mention it, I remember hearing the same thing.

Unfortunately, it's a common problem with 2002 and earlier 5.4L engines. In those engines, the heads only hold the plugs by about 4 threads -- not nearly enough! Lightnings in particular are known for launching their plugs (I didn't know it also happened on the naturally-aspirated engines). The problem has been corrected on the 2003 and later heads by increasing the thread depth to 8, so replacing the heads with new ones usually solves the problem.

-- Jim

Actually three full threads are sufficient to develop maximum strength.

Ed

Ed,

Where are you getting *that* information. Clearly 4 threads is insufficient.

insufficient.

Actually, threads reach near full strength capacity at about 1.5 times the thread diameter which would be 21mm for most spark plugs. I believe the thread pitch on a plug is 1.5mm so "maximum" strength would be attained at

Chris

Maybe the guy who designed the Ford heads had the same source of bad information as CE White! However, common sense should prevail that 4 threads ain't enough.

There was a sig file something like this...

"in theory, theory and practice are the same, in practice they aren't" -?

Seems to me, in theory;) a certain accuracy of the treads would have to established first before the minimum number could be decided on.

Alvin in AZ

Alvin,

Most common threads are known as class 2 fit. Precision threads such as spark plug threads are usually class 3. There are actually class 4 and I believe 5, which are interference fit threads. If the threads are made right to the middle of the tolerances, there is very little difference between the maximum strengths of class 2 and 3. When a thread is machined, it is not actually a full thread that is peaked out on the top and bottom of the threads. Under magnification you can see a small flat at the crest of a male thread. The same holds true for a female thread, it is just harder to see because it is inside the hole. Most threads are machined to 65-75% of full thread. This means that if you measure a 3/8" bolt, the threads will actually be about .371 diameter. There are 2 reasons they do this. First, there is very little strength increase over 75% thread, and second, threads with a higher percentage of full thread are very difficult to manufacture, especially in the internal threads. The tapping force required increases dramatically for very little gain in strength. These numbers are all set standards in DIN for metric threads and SAE and others for standard threads.

Chris

Clear to who? Spark plug threads into an aluminum head are a complicated stress problem. Some of the other respondents mention a thread engagement length of 1 diameter or more. These recommendations are for bolts where the desire is to have the bolt pull in two before the threads are sheared. This has little to do with what is desirable for a spark plug. I am certain that four threads are more than sufficient to properly retain a spark plug that is properly torque. The difference in the stripping torque between 4 threads and 14 threads is not going to be as significant as some of you seem to think. Without specific information on the properties of the head material, the spark plug material, and the thread forms I can't give you the actual torque that it would take to strip the threads, but I am confident that it is well above the recommended tightening torque of the spark plugs.

BTW, after checking, I found that the 5.4L spark plugs are 14 mm x 1.25 thread. For a bolt with this thread, the standard short engagement length is 4.5 mm. This length includes the threads and lead in chamfers -which amounts to three full threads. The standard long engagement length is 13 mm. These two engagement lengths are the boundaries of good engineering practice for standard threaded joints.

Ed

Using your theory an 18 mm plug needs 18 mm or more of threads, yet plenty of older engines used 18 mm spark plugs with only four full threads. Tapered seat spark plugs don't stress the threads nearly as much as plugs with traditional gaskets (at least if they are properly torque).

It would be interesting to know how many spark plugs blew out of engines which still had the factory installed plugs.

It should also be noted that the "bad" heads actually have 5 or 6 threads, not

1. This is more than enough threads to resist stripping if the plugs are properly torqued. If they are not properly torqued, well there is no reasonable number of threads in an aluminum head will prevent stripping. But I suppose you can argue more is better - at least in this one respect.

I have no idea of the real reasons that some modular engines have had spark plugs blow out (but I know it has happened). I am sure that 4 threads is more than sufficient to provide adequate retention of properly torqued spark plugs assuming the threads are properly formed and that the plugs are properly installed.

Ed

I've had spark plugs get loose in old aircooled VW heads before and the threads in the heads were messed up and causing the situation to get worse. I figure, somewhere, sometime, me or the previous owner did something wrong to cause the trouble. "Rebuilt" heads can come that way too. :/

This reminds me of back when I used to maintain railroad power switch machines and the bolts that connected everything to the switch points.

If a nut got loose the threads could be ruined both inside the nut and the bolt in just a week. Sometimes a track department gang had to pull apart the switch point so certain bolts could be replaced. It was an on going process trying to keep all those bolts tight.

I got around all that by leaving off all the lock washers and any other washers that weren't absolutely neccesary and double nutted everything instead. I found the bolts etc lasted as long as the life of switch points (5 to 10 years) when done that way. I was the only one doing it that way and couldn't convince anyone else to even try it. :/

The ball on your pickup has one nut and a lock washer right? ;)

If your ball has 3/4" coarse thread, get rid of it and buy a 1" fine threaded ball and buy an extra nut to go with it at the same time. Drilling out the bumper to 1" will be worth it in the long run too.

Alvin in AZ