Torquing Spark Plugs
I bought spark plugs the other day and they had a very general chart with torque specifications on the back of the box. The box has columns for Iron Heads and Aluminum heads with installation torque specifications for the three spark plug profiles in the line, but they also include a “Hand Tight +” column. Completely aside, stare at the chart for a few seconds and see if you can see why there are three rows of values inside each cell — pause for effect — it’s because they’re repeated for English, French, and Spanish: “18–25 lb/ft” is, of course, exactly the same in each language.
Torque is a pretty widely understood metric for fastener installation and it’s easy to use — set the torque wrench, wrench until tight, done. It’s also easy to digest as a concept, the fastener is being tightened as though there was X lb on the end of a Y ft level being drawn down by gravity. So why the third column? And why is there only one third column while there’s two for torque?
The naive interpretation is it’s for people without torque wrenches, but it goes much deeper than that.
Think about what you’re actually asking the threaded fastener to do, barring some very specific cases (interference seals), you’re either asking the fastener to clamp two things together or you’re incorrectly using a bolt as a shaft-pin. In the most common clamping case, what you really care about is how tightly the bolt is holding the things together — which is a measure of the bolts preload. Effectively the bolt is a really, really stiff spring and the threads are used to extend the spring.
Torque is simply an easily measured metric for clamping force, but the relationship isn’t as strongly co-related as you’d hope. There are several factors which contribute to the measured torque (resistance to turning) and at typical tightening loads, only a small fraction of that is preload/elastic-elongation — this means the surface conditions can have a greater effect on measured torque than the preload we were trying to measure to start with.
The difference in surface conditions (coefficient of friction and finish quality) is why the spark plug packet has both cast-iron and aluminium columns for torque. With the direct-rotation measure, the elongation of the fastener is directly related to the number of turns regardless of the surface conditions. Assuming the substrate’s yield strength is negligible, 1/4 turn creates the same elongation in aluminium, mild steel, or over-cooked toast.
Assuming you can’t directly measure the clamping force (Strain Gauge), indirect measurements are progressively less accurate. The torque wrench’s 25% initially appears to be a large error range but it’s well within the bounds of the up-to-100% torque ranges given in common settings (e.g. 7–14 Lb-ft).
So what is “Hand Tight +”? The angle Torquing concept is based on the assumption that fastening torque has a sharp-increase after the fastener seats and the required torque changes from threading resistance to threading resistance plus elongation strain plus surface friction with all of those being non-linear with increasing clamping force. Given the sudden increase in resistance to torque, it should be possible to thread the fastener to a “reliably consistent point” and then rotate the fastener by another X fractional rotations to generate elongation.
In the context outside of spark plug fastening, the Hand Tight + is often replaced with “Seat + Flats” where the fastener is Seated flush with the surface and then rotated by a certain number of “Flats” to get a reliable insertion. That’s often not applicable for Spark Plugs where the spark plug is installed deep in the head below visibility. In this context the concept of “Hand Tight” needs to be balanced against common sense to overcome any friction in threading.
Why do we worry about this in the first place? Spark Plugs are exposed to the entire cycle of the cylinder, so they need to reliably contain the compression. The spark plug seals either by crushing a metal washer-style gasket against the head or by a tapered seal against the head directly. In either case, the clamping force between the spark plug and the head is required to generate a good seal and that clamping force is generated by threading in to the head.
To a lesser extent, it’s important to avoid over-torquing and damaging the block threads or fracturing the spark plug inside the head. Both of which are generally beyond reasonable to repair.
Other miscellaneous whining — Pound-Feet of torque is a composite unit not a relational unit, but typesetting it as “Lb/Ft” is depressingly common.
