This is such a terrible topic. The physics and mechanics are so poorly understood by damn near everyone, and the verbiage is so close that it's SUPER easy to get confused on what is actually being said. SO......when that guy says "Don't put lubrication because it increases the torque loading" (right around 32:20 or so), I can tell he either doesn't understand WTF he's talking about, or at least doesn't understand the necessary verbiage to use to communicate it well. Using Anti-seize doesn't "increase torque loading"; the torque loading is what the tool puts on the fastener. That is the only way I know to interpret that set of words, and the tool operator has complete control over that.
I design customer fasteners for high load applications on a semi regular basis. I have to write I/O manuals for vibratory equipment that contains bolted connections and has to explain this concept, and people still get it wrong. Maybe I'm not a great writer, who knows. My most recent design involved operating hours, thermal cycles, and a material creep properties to keep a 1,200degF machine running at a shade over 4g's of vibration together with a bolted connection. You can get in the weeds in a hurry here, and once you're in the weeds you lose sight of the end goals. I've had to field service these machines that develop massive loads on bolted connections and create life safety issues if they aren't done right. Not to completely pat myself on the back, but of all the things I think I know, this one is WAY up there on my confidence scale, having climbed over the Dunning-Krueger curve literally decades ago.
What I have below is an attempt to put some information out there that can be referenced in the future. While I have extensive experience in bolted connections, I'm NOT the engineer or test tech that determined the proper torque for our Yamaha engines, nor am I the one who declares if you should or should not use anti-seize on your spark plugs. Please read through the info below, and make your own, hopefully now more educated, decision. My practice is listed below, like everything else you read on the internet, your mileage may vary, and I'm offering no warranties.
In a bolted connection you want to target a clamp load. That's the goal. How much force along the axis of the bolted connection is being imparted. This is EXCEPTIONALLY difficult to measure in the field. SO, instead you design a fixture in a shop that can measure that clamp load, do lots and lots of testing, and develop a curve fit equation that relates torque applied to tension force along the axis under a given set of conditions. Within the formulas that are used to predict the relationship exists a coefficient of friction. This MUST be laboratory tested to have any idea what it is. You can google for weeks, and read books for months and not find agreeing data. SO, you MUST have some idea of what the joint looks like, how it's intended to operate, and how much clamp load you need before you can even begin to specify a torque. Even then it's most likely wrong. Anything beyond that is empirically measured and tested and that curve is developed.
Beyond just that, there are other methods such as "turn of the nut", and "Torque to Yield" applications that can be used to get a more consistent clamp load. These are often used on cylinder heads and other highly (and cyclically) loaded joints. I won't go into that here, there are PhD level dissertations written on this and in general you have to be beyond neck deep in a project to need this level of detail in your joint design.
What REALLY happens is that you can apply LESS torque to achieve the same clamp load when you lubricate a fastener. If you apply the same torque to a lubricated fastener the clamp load will be HIGHER than with an unlubricated fastener. This is due to the coefficient of friction between the male and female pieces being LOWER. As you apply torque, some of that rotation force is used to overcome the friction in the threads, lubricate it and less of the force is used to overcome friction and more is used to increase clamp load (by straining the bolt more).
We should also talk about the use of Never-Seize, and what it's purpose is. It's purpose is NOT to lubricate the threads, well, not in the sense of lowering the friction constant anyway. It's purpose it to provide a barrier between dissimilar metals, and to prevent corrosion. We use it on a regular basis in high temperature applications, as well as anytime we have dissimilar metals. The high temps will corrode a fastener quickly, and removal will often gall the threads, likewise stainless on stainless is know to gall easily, so we will use it there, especially in high clamp load situations. With all that said, it does lower the friction constant, and thus will produce a higher clamp load per unit torque applied to it. At some level, the video is not wrong, but WOW it's pretty misleading in his verbiage.
Finally, we need to talk about accuracy of clamp load in regards to torque application. Because of the myriad of details that go into determining that relationship, it's WILDLY inaccurate without laboratory testing. If you change a single variable then all of the testing is invalidated, and you're back to essentially guessing at clamp load. Engineers edge publishes a table that reports +/- 30% for unlubricated and +/-25% for lubricated fasteners when comparing actual clamp load to calculated clamp load.
You can find that here with some additional reading on how clamp load is developed.
Here is another good link from Engineers Edge. It has some coefficient of friction values listed below. You can see the difference between a stainless and a lubricated bolt is only about 8% or so, still well within the 50% (+/-25%) accuracy window.
Even when I was developing custom bolt tensioning devices, using a strain gage to measure clamp load on machined surfaces and diameters, I had error in the +/- 5% range. With that range of error, and knowing how I would develop that spec, I would bet money there is empirical testing behind the torque value listed in the owners manual. For reference, the below pictures show the amount of work it takes to strain gage a bolt under tension. I had the center section machined to a precise diameter and finish, applied a rosette strain gage, then ran through multiple torque values taking strain measurements along the way. Once all of that was in place, I developed a torque to strain curve, and calculated bolt tension (and clamp load) from there. The only way to be more accurate than this is with significantly more expensive load cells and the increased margin of precision is very slim .
View attachment 172026 View attachment 172027<--click for a larger pictures of my office desk.
To wrap it all up........How does all this apply to spark plugs? They're different from a bolt and a nut, or a bolt and a blind threaded hole right? YES, they are different. The forces that a spark plug endure are cyclic in nature, and relatively high. You often have dissimilar metals (stainless, or zinc coated plugs into an aluminum head), and there is often a crush washer involved that creates a gas tight seal. It's a complicated setup as far as general fasteners go, at least in terms of those shade tree mechanics have to be involved with. The important aspects are to make sure we have enough clamp load applied to deform the crush washer and seal the combustion chamber. We want enough clamp load to "self lock" the threads at some level and prevent the plugs from backing out under moderate to high vibration levels. Finally we want to make sure we can take them back apart and replace them, since they are wear items. The use, or disuse of anti-seize in this application is in the margins of "victory" for making those three things happen. The clamp load is achieved at a far LOWER torque than you think, and the room for overhead is far HIGHER than you would expect. Point being, if you put the plug in the hole, it threads properly, and you use a torque wrench (that's calibrated right?....you know that's another HUGE source of error right?) then you'll achieve enough clamp load to not have a problem, and not achieve enough to do damage.
Seriously, it really doesn't matter on our engines. With all the knowledge and expertise I have on threaded connections, and all the understanding I have about that fastener.....I use Anti-Seize, and a 3/8 drive ratchet and feel. I haven't used a torque wrench to put plugs in for years, and doubt I ever will. You can feel when it seats, you can feel when the crush washer has been crushed, and then a bit more rotation is added to ensure they're "good-n-tight", and I'm done. The anti-seize ensure they come back apart when I need to put the next set in. I would wager I'm over-torqueing them slightly, as I think we're all a little stronger than we realize (or more accurately, we develop torque a bit easier than we think), however I'm clearly not damaging the threads, and they joint is still only semi-permanent. I've found a process that lands me in the rather large sweet spot between "just enough" and "too much".
As an interesting anecdote; those Ford heads that had the spark plugs seize/strip threads, were exceptionally poorly designed. I've personally put Heli-Coil fixes in 6/8 cylinders in an old Ford Van. The heads had less than 1.0x diameter of threads (in terms of length), which is just barely enough on lightly loaded application, much less a fatigue prone cyclically loaded joint like a spark plug. You almost can't torque something like that properly because it's designed so close to the edge of acceptable from the onset the propensity to miss that "sweet spot" is very high, because the target zone is so small between holding and damage.
