Removing Old Spark Plugs....
 

Question...

Is it easier and/or safer to remove seized spark plugs from a warm engine or a cold one?

I remember years ago snapping off two plugs in my old 164 Volvo, and it was not fun. I ended up removing them with an "easy out". I'm hoping to avoid that type of event...

Thanks,
Paul

Have you tried them yet? Maybe you're worrying for nothing...

If the head is aluminum, they should come out easier when it's warm. If they're sticky, you can spray some penetrating oil at the base and work them back and forth a little.

Beg to differ.

On an aluminum head you want the engine stone cold when pulling the plugs. Heat expands the metal, which makes a tighter fit. Pulling plugs on a stone cold aluminum head minimizes the chances of pulling the threads out.

If they are already siezed, I guess anything is worth a try as you'll end up having to helicoil the plug holes anyways. Use anti-seize and a torque wrench when installing spark plugs.

Mike,

I'll go with you on this, after a little web research. Conventional wisdom says to remove the plugs when the engine is cold.

Help me out though. When heating metal, all the dimensions grow, including the diameter of the spark plug hole. Since aluminum expands more with heat than steel, the hole gets bigger with heat. The plug grows too, but not as much.

Well, the flip side is that the head gets thicker too. That would increase the torque needed to break the plug loose.

A final argument FOR Mikes case is that engines are assembled cold. So the clearances and torque specs must be designed accordingly.

Good, I learned something!

Actually, as the aluminum expands the hole gets smaller.

Thanks,

I did use some penetrating oil and allowed it to work overnight. Today I was able to remove the plugs with the engine cold.

My previous experience has left me a little gun shy........... :D

Mike,

It's back to Physics 101 for you! Please do a little research and you'll find that you are incorrect.

For example:

http://wug.physics.uiuc.edu/courses/phys101/fall01/Lectures/Lect21.ppt

See page 7.

:)

As i see it Mike is rigth. The hole is empty, what is gonna expand is the AL sorounding it, so if that is the case the hole is gonna get smaller, on the other hand the plug, filling the hole is gonna get bigger. If it was my car i would wait for it to be stone cold.

I agree to remove the plug with the engine cold.

However, you guys can't change the laws of physics. The hole gets bigger. Check it out by doing your research. Your intuition is leading you down the wrong path.

300EE320 is right. This is the type of misconception I try to clarify everyday (I teach physics at a college).

The diameter of a hole will always get bigger if heated for materials that have normal thermal expansion properties (99% of the materials have normal thermal expansion properties).

Think of this way.

If you have a shaft and a bearing and the shaft won't fit into the bearing hole (by a little bit), what do you do? You heat up the bearing so the hole is large and therefore the shaft can go through.

If the plug (steel) and the hole (Aluminum) are a tight fit when cold, heating both will make it easier to remove the plug because aluminum expands more than steel (twice as much).

Stand corrected!

Thanks Loubapache. If push came to shove, I would have wagered my Ph.D. (Physics and Applied Mathematics) on it! :D

I can think of other reasons why you would want the head cold (in addition to those I listed above). Since hot aluminum has a lower density than cold aluminum, it would be more likely to be galled by the steel plug.

Also, depending on how hot we're talking about, you wouldn't want the cool air rushing in and hitting a red-hot exhaust valve.

So, there are probably lots of reasons, but it sure isn't the diameter of the hole!

Well, I'll admit I don't know diddly about physics so I guess I have some reading to do.

But, I see it like this, and I guess that's wrong, as the aluminum expands the external dimensions get larger (however miniscule), as the dimensions get larger, the hole through the material can only get smaller. The expanding material makes the diameter of the opening smaller thus tighter. If the hole actually was increasing in size as the explained laws of physics suggest, then on a hot engine the plugs would actually be looser, not tighter, and the threads would not be damaged - contrary to real life.

OK, time to read.

EDIT: will have to wait till I can read the link at work as I don't have powerpoint.

Well, my Ph.D. is in classical Greek and Latin Literature, but
 

I earned it partly by working on some very expensive aluminum motors (and big cast-iron diesels) almost 40 years ago, and this is what I would recommend from dirty-fingered experience rather than theory:

1. Warm up the engine thoroughly the evening before you intend to work on it and spray several doses of penetrating oil (nowadays my preference is "PB Blaster") on the plugs while everything is warm enough to promote the maximum chemical interactions between the penetrating oil and the corrosion.

2. Next morning, when everything is cool, give it another shot of penetrating oil and then get out the tools.

PS. my nom du keyboard is Latin for "Dung Develops" or "Excrement Occurs," or however else you want to translate it.

Logic, not physics, would lead me to believe that the "hole" gets smaller as the metal expands.

I equate it to the blow up life preservers they sell for children. You slide the preserver deflated over the childs arm and then blow it up - expanding the device making the inner diameter smaller, therefore securing it onto the child.

?????

I used them on 3 of my kids and no one drowned... guess they work. ;)