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  #16  
Old 03-27-2002, 06:21 PM
Dalcorn
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Regarding the hole size, opposing forces at work here. Last night I had a hard time opening a jar of pickles that was in the refrigerator so I ran hot water over the cap and it twisted off easily. The heated metal in the cap expanded in all directions. One result of the expansion was that the circumference of the cap increased resulting in a bigger hole. The circumference was able to grow because the expansion displaced air that provided little or no resistance.

As an experiment, lets embed the outside of the cap in concrete prior to heating it up (assuming that concrete does not expand in the relevant heat range). Upon heating the cap, the metal would still expand but the expansion would be constrained by the resistance of the concrete. Therefore, the expansion would be directed inward, resulting in a smaller hole (and the cap may go out of round).

Regarding a spark plug hole, an aluminum head is not like either air or concrete. It is metal that expands and is malleable. The notes to Lecture 21, Pre Flight in the referenced web site is cute but is intended to highlight a difference in the relative rate of expansion in aluminum versus copper. I would not bet the farm that it provides definitive guidance that a spark plug hole would either expand or contract.
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  #17  
Old 03-27-2002, 06:29 PM
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pjtib,

Your analogy does not match the laws of thermal expansion. This problem is a very popular one for college physics exams. The students who studied for the exam get the question right. Those who try to reason it out by comparing the problem to something else usually get it wrong.

David,

It is a good point that the head is constrained by the headbolts. I'm sticking with my assertion though.
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  #18  
Old 03-27-2002, 07:08 PM
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This is turning into a very interesting thread.

The Laws of Physics definitely predict the metal will expand when heated. In most simple geometries the diameter of a hole will expand, as will the diameter of a shaft and so on. The geometry of a hole in a shape as complex as an aluminum cylinder head will change according to the Laws of Physics applied to the entire volume of the head, taking its internal and external features and shapes into consideration. It is not clear that the diameter will expand evenly or consistently for the entire length of the hole though.

In the end the hole for the spark plug will change dimensions, but before I would bet a lot of anything on the outcome, I would either check it with a guage hot and cold, or I would run a finite element analysis to identify how the shape changes as it heats up. Any relief you get from the diameter growing will likely be offset by the length change, making the threads a slightly different pitch, kind of like an interference fit. In addition, the mechanical properties of aluminum degrade with temperature, and a head at normal operating temperature is more likely to let go of the threads than it would be at room temperature.

The primary argument to change the plug cold is the engine was designed to be assembled at room temp and should be disassembled at the same temp. Any tricks of the trade, like adding penetrant when it is hot to help draw it into the thread are great to know, so thanks for the tips. Jim
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  #19  
Old 03-27-2002, 07:14 PM
Dalcorn
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After reading the linked explanation of linear thermal expansion, I am convinced that heat will enlarge the hole.

http://www.physics.umd.edu/deptinfo/facilities/lecdem/QOTW/a057.html
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  #20  
Old 03-27-2002, 08:50 PM
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I must agree with the Professor... the shaft and bearing example is totally correct. I work with quite a few bearings and shaft combinations (on 1000Hp + electric motor and pumps) and have had to heat up many a bearing to get them on or off. I don't know but I'd bet there is differences between inflation (pressure) and expansion/contraction. Any that's my 2 cents... and probably not worth that! Nonetheless good luck with the plugs
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  #21  
Old 03-28-2002, 02:03 AM
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Physics 101

An interesting lesson. I won't say I completely understand the principle as I was an Ag major at a little Agricultural & Mechanical College in Texas, and as such only had to deal with Cowboy Calculus.

I guess I just wasn't looking at the "big picture" but rather just looking at the hole using logic. And to think all these years I believed it was the contraction of cooling that was the break free force when you heated a stuck bolt/nut, I guess I had it backwards.

That's where I always messed up in math - got the answer right, but was marked wrong for not showing the work, or having used the wrong formula.

Gotta love the the direction some of these threads go. Thanks again for the lesson.
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  #22  
Old 03-28-2002, 10:45 AM
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Remove Plugs Cold

According to the following site;

www.jonko.com/forum/tutorials/basics/plug.htm

Spark plugs should only be removed when the engine is cold.
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  #23  
Old 03-29-2002, 12:53 AM
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. . . mmmmm all very interesting, but my service manual tells me when doing a compression check, to warm the engine up first then remove the spark plugs !
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  #24  
Old 03-29-2002, 09:18 AM
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Remove spark plugs cold!

There are many sites on the web that confirm that spark plugs should be removed when the engine is cool. When doing a compression test the enging must be warm to insure that the engine pistons, and other parts have expanded, and distorted, to their operating condition. The only way to do this to run the engine to operating temperature and remove the plugs and complete the compression test as fast as possible before things cool down too much.
If the plugs have been in the engine a long time (more than six months), I think I would remove them when the engine was cold and put a very light coating of anti-seize on the threads and reinstall them with a torque of about 10ft-lbs. Run the engine to warm it up completely and than remove the plugs for the compression test. This should insure that the cylinder head will not be damaged.
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  #25  
Old 03-29-2002, 09:36 AM
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Wink

Hi Tom,
Yes, I would agree with you in principle as the service CD is totally useless when it comes to the bit with the compression test, I did mine a few days ago on the 500SEL (117 engine) and just used some sound engineering practice. I remember changing the plugs some 10000 kms ago and then use anti seize compound and cranked them up to the book figure of 25Nms. When I tried to gem them out this time, oh boy, were they tight, and all gummed up too ? I will never again use antiseize grease on an alloy head. I thought that the dissimilar metals might give me problems when it came to changing them, and it just made it worse ! The 117 engine plugs a re a bit tricky to get at quiclky as I had to use a different combinations of 3/8" drive extensions and universals. By the time I had finished, the engine was fairly cool and this time I replaced the plugs nice and clean and wrenched them up to the higher end of the torque setting. I guess this debate could go on forever, just depends on your choice and experience. I do agree though that you need the engine as near as you can to it's normal operating temperature. ( just as you do when you change the oil, eh Larry ?
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  #26  
Old 03-29-2002, 06:37 PM
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I looked around on the Internet too and found many places that say "remember, NEVER remove plugs from an aluminum head when hot".

Well, I think the jury's still out on this one. I'm believing more and more that it's an old wive's tale.

Here's more real world evidence:

Drag racers (with aluminum heads) read their plugs between every round. Should we suggest to them that it's a bad idea because we read something on the Internet? How about every other kind of racer? They don't let their cars sit in the pits all night so the engine can cool. They dig in and start pulling plugs to see how things are looking.

I'm also an avid dirt biker (alum head). The most important part of setting up a new bike is jetting the carb. This involves running under load at fixed throttle settings and pulling the plug to read it. It's common to pull the plug 20 times within a few hours while tuning. This is done on every new bike, and it's done repeatedly to tune for different elevations.

I believe that the most common problem with plug threads in aluminum heads is due to cross threading and over-torqueing. Just my position on it, but I'm going to continue pulling plugs while warm.
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  #27  
Old 03-29-2002, 07:37 PM
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I suspect a lot of hanging and dimpled chads:p
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