Overtightening spark plugs in aluminum heads...

A foot pound is the same as a pound foot. I think the reason for putting an order to the units was to keep the novice student from being confused by torque, work, and energy. The units for torque and energy (or work for that matter) are the same; the meanings are different. There are plenty of topics for beginning engineering students to get confused about. An attempt is being made to avoid confusion. Pound feet provides a clear handle for the novice to grasp and the pedant to use.

The fact that a torque is a vector quantity (r cross F) has nothing to do with its units and which way they can be written. That is a red herring. If you take the order they appear in the evaluation (rFsin(theta)), you have to write ft-lb.

Again, it is either a foot pound or a pound foot.

 

The reason for expressing the units in a particular order is to avoid ambiguity. PSk asked which was the correct order when referring to torque, ft-lb or lb-ft. I have written that it is customary in the U.S. when referring to torque to write "lb-ft" and when referring to work or energy to write "ft-lb." I stand by that assertion.

You have mentioned that "it is semantics and personal preference only," implying that no convention exists as to the order of the units, and alternatively that "the reason for putting an order to the units was to keep the novice student from being confused," implying that there is, in fact, a convention. Well, there is a convention, just as PSk suspected, and everyone (not just novices and pedants) is free to follow it.

My bringing up the fact that torque is a vector quantity is not a "red herring." It goes to the heart of the matter. As you know, the magnitude of distance vector X projected onto the plane of force vector Y (work) is not interchangeable with the magnitude of the (torque) vector represented by the cross product of moment-arm distance vector X and the perpendicular force vector Y. But when you cancel everything out, you are left with force and distance in both cases. How do you indicate what you mean?

Well, most of the world indicates what it means by saying "joules" (newtons x meters) when referring to work and "newton-meters" (also newtons x meters) when referring to torque, but the custom for U.S. units is, respectively, "foot-pounds" and "pound-feet," because there is no special name for the U.S. expression of work. Surely the practicing engineer (or rank amateur, for that matter) who prefers "foot-pound" when referring to work or energy is no more pedantic than his counterpart who bothers to speak in terms of "joules." It is a useful convention.

 

Ken - I know that Phil Woods grease is great stuff, but I don't know if it is made to withstand the high heat of the cylinder head. The Mondial 3.2 owners manual states that the threads should be lightly coated with "graphite grease". I use Permatex Anti-Seize Lubricant -- I'm not sure if this is graphite based, but it is designed to withstand the heat. I did also confirm that the correct procedure for new plugs (on a 3.2) is to initially tighten to 15 lb-ft, then slacken, and retighten to 12 ft-lb.

 

I have a more intersting question than lb-ft versus ft-lb. Is it okay to use a 3/8" socket on a 1/2" torque wrench using a 1/2"-3/8" socket adapter -- or does that throw off the torque reading? This is mostly hypothetical for me because I have both size torque wrenches.

 

Good point about the heat. There is no burnt residue on the threads, but next time I'm at Autozone I'll spring for the "real" stuff!

Ken