I used a scrap 2001 model year PI 2V V10 head with good threads to test the breaking point of the plug threads that are so weak according to many people on this forum.
I did four plugs, two with antiseize and two dry.
I started with the correct torque ( 180 inch pounds ) and worked up.

At 55 ft lbs it felt like the threads were just starting to stretch.
From 55 to 85 I got about 11/2 turns more out of them, and it did not feel good.

Two of the plugs snaped off flush with the heads just shy of 100 ft lbs, after anouther 3/4 turns from 85 ft lbs. One was dry and one had anti seize.

The third plug broke right at of 100, 5/8s of a turn after 85 ft lbs. This one had anti seize.

The forth made it to 115 ft lbs, 1/8 turn past 100 ft lbs ( 7/8s of a turn passed 85 ft lbs) and snaped. This was a dry plug.

Every one of them the plug broke, I never striped the threads out of the heads like I though I would.

 

Nice job on the testing, Bill. Great information!

 

I would have thought the threads would have come out of the heads. Go figure.
Anyone know if my 2010 6.8 has any differences than the earlier heads?

 

Good Job Bill. So I guess 4 threads is enough to hold the plug in there and going to 20+ ft-lbs is probably not going to hurt anything. One other thing that might be of value is to drill out what is left of the plug and see what the aluminum threads look like after 100 ft-lbs. Usually three threads gets you 100% holding power on a bolt but it is not optimum. Usually, especially in soft metals, you want the thread length to be at least the diameter of the bolt.

Perry

 

Completely different spark plug design on your 2010 3V than the earlier 2Vs. Over the last decade+ the modulars have gone through several spark plug and spark plug hole related changes.


Good info Bill. You confirmed what I often thought would be the case. (threads handling a lot of torque) I wasn't exspecting the spark plugs to break as easy, it just goes to show the weld on the plug is not as strong as some would think and goes to show why the old 3V plug design had with 2 pc plugs had breaking issues.

 

Thanks.

If its in a van than its a 2V PI head with ( I think ) 12 threads. They are the least troublesome from what I have read.

If its in an F-series than its a 3V head with more than enough thread.
They are known the not want to come out. The threads stick down into the cylinder and deposits build on the threads. Its not uncommon for them to snap off during a change. There is a tool that gets them out with out pulling the head though.


I pulled the head and spent an hour this morning trying to remove one and had no luck. Did see something intesting though. Some of the plugs had gone deeper than others, they were not all into the combustion chamber the same amount. They had all gone at least 1/8" farther than the 5th plug, which was set at 14 ft lbs.

 

Of the 4 plugs that broke, they broke in 3 diffrent spots.

2 snaped in half flush with the head, leaving the top half with the hex head in the socket and the bottom half threaded into the head.

1 left just the threads in the head, and every other part of the plug came out including the center than runs through the threaded part.

the last one only the nut broke off the plug, all the rest of it is intact in the head.



This engines about to be gone, but I will have others to do this with in the future if anybody sees something I missed or thinks of something to add.

 

That is because you stretched or yielded the metal before it broke. The part between threads and the seat is what is stretched till it broke. Some stretched more than others before they broke and that is why some are deeper. Soft metals will stretch a long way before breaking but they will yield sooner. Harder metals tend to not yield so soon but when they break there is less permanent stretch. You can see in the picture below that the aluminum stretched alot farther before breaking than the harder steel specimen.

http://www.mse.mtu.edu/outreach/virt...el_Samples.jpg


Perry

 

Its an F350. So what interval for the first plug change.
50 000kms?

 

Try 161,000 km or 100,000 miles. Main thing is to keep water out of the plug chambers.

Perry

 

The only thing missing from this test is what happens when the engine is running and both the head and the plug expand from the heat.

I suspect that after a few hundred heat-cool cycles that an overtight plug could eventually pull the threads from the head.

There's two things going on - one is rotational stress as you tighten the plug which makes the plug break flush with the head. The other is when the head expands and the aluminum expands much more than the iron/steel plug.

The coefficient of thermal expansion for aluminum is more than double that of cast iron, and almost twice that of steel.

If the plug is torqued down too far, and the aluminum expands more much than the iron/steel plug, it WILL either pull the threads right out of the head, or more likely, hog out the angled seat in the head, which loosens the plug right up.

The more torque, the more the angled seat will get flared out over time.

 

I looked into thermal expansion. We are talking .0003 inch here over the distances we are talking about. The thermal expansion coefficient of Aluminum is about 1e-6 in/in per degree F. Steel is about half that. So lets just say the heads get to 300 F. The the difference over an inch is about half of .0003 inch since steel expands about half that much. So we are talking .00015 inch change in the length of the plug relative to the head over 1in. There could be a fatique problem with the threads though. The combustion process puts stress on the threads as well.

Another interesting test would be to put one of the heads in the oven at 300F and then do the torque test with the head hot. The higher the temperature the weaker the aluminum is going to get.

Perry

 

As Art and Perry indicated, your test does not replicate real world conditions. Just to say that the head will withstand significant overtightening sitting cold does not indicate what will happen if the plug is overtightened and then goes through several heat cycles. Also What happens when the overtightened plug is removed and another plug is installed, even if installed correctly, it may not hold as it should due to damage that may have occurred when the first plug was overtightened.

 

No but it will put to rest the don't tighten over 14 ft-lb myth or the threads will strip off myth. Also remember the plug failed not the threads. I expect 90% of problems occur when the plugs become loose and you get hot gases heating the threads as well as wiggle every time the engine fires. Structurally I don't see that the thread is not sufficient for what it was designed for but if you cross thread or otherwise damage the threads you may have problems. If you put a plug in and it does not screw in easy chances are that at least half of the specified torque is used up just turning the plug before it seats. Now with a clean lubed thread, all your torque is going into tightening the plug. I am going to check mine in a year and see if any have loosened up.

 

i think the only thing he was trying to prove was teh myth of any amount of overtightening with the early non-PI heads would pull the threads clean out..

i think his examples clearly show even the worst heads on these motors have more then enuf strength to hold teh plugs in if checked and tightened properly..

ps THX bill for being nice enuf to test a set of heads out.. 3 cheers..