Fastener Torque

Fastener Torque Specifications

The smarter you are, the more often you'll use the torque wrench.– Ed Sorbo, Lindemann Engineering

Tighten until it strips, then back it off a quarter-turn.– old mechanics' joke

Long ago I found the following in Machinery's Handbook regarding accuracy when torquing fasteners:

By feel +/- 35%
Torque wrench +/- 25%
Turn-of-nut +/- 15%
Preload washers +/- 10%
Bolt elongation +/- 3%
Strain gauges +/- 1%

My introductory quotes notwithstanding, torquing by feel is not that much worse than using a torque wrench. But it assumes you have used a torque wrench enough to develop the “by feel” skill. And I would certainly never torque something critical, like connecting rod bolts, by feel.

Of course, you also have to use some common sense to detect typographical errors. (As when Sherco said to torque their clutch center nut to 6 Nm when it should be 60 Nm.)

Some general comments: The threads must be clean for the torque value to be meaningful. A lubricated thread requires less torque to achieve the same clamping force as a non-lubricated thread. Aluminum threads are not as strong as steel threads - this is especially true of aluminum castings. Don't confuse the diameter of the thread with the tool size needed to turn the fastener.

EM provided a torque table in the owner's manual. Unfortunately, some important components have no specification. Some translations could be improved. The French word “carter” is used without translation into English. It means crankcase, but gearcase would be more correct for the application.

There's an additional complication if your torque wrench is not calibrated in newton-meters. My attempt at an improved torque table is a spreadsheet available for download. Some of the values may be refined over time. There are separate sheets for the 5.7 and ePure.

EM Torque Specs.ods

Foot-Pound or Pound-Foot?

My torque wrenches and I are old enough to be calibrated in foot-pounds. But that's no longer considered correct as the foot-pound is a measure of energy, whereas the pound-foot is the unit for torque. (And even more correct is to state that it's pound-force, as opposed to pound-mass, or just pound.)

Although either word order gets the point across when talking about fasteners, you'll be subject to ridicule by pedantic scientists and engineers when you get it backward.

Frankly, it's easier just to use newton-meters, and I'm moving in that direction. But even the proper written form of that is subject to disagreement. Some authorities put a space (or even more correctly a half-high dot) between the N and the m.

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