Torque Converter

Torque is rotational force, the twisting effort that turns a bolt, spins a wheel, or cranks an engine. The SI unit is the newton-meter (Nm); the imperial world uses the foot-pound (ft-lb).

Key relationships:

• 1 ft-lb = 1.35582 Nm
• 1 kgf-m = 9.80665 Nm
• 1 in-lb = 0.112985 Nm

Common references:

• A car lug nut: 80–100 ft-lb (108–136 Nm)
• A spark plug: 12–18 ft-lb (16–24 Nm)
• A bicycle pedal: 30–35 Nm

These figures matter more than they look. Under-torque a lug nut and the wheel can work loose; over-torque it and you warp a brake rotor or snap the stud. That’s the whole reason torque wrenches exist, and why mechanics care about the exact number rather than “good and tight.”

The classic mix-up is torque versus power. Torque is the turning force at a given instant; power is how much work that force does over time, which depends on how fast the shaft spins. RPM ties them together:

• Power (HP) = Torque (ft-lb) × RPM ÷ 5252
• Power (kW) = Torque (Nm) × RPM ÷ 9549

That’s why a diesel can make huge torque at low RPM yet modest horsepower, while a small high-revving engine does the reverse. They’re two different questions about the same spinning shaft.

There is a neat consequence buried in that 5252. Because horsepower was defined as 33,000 ft-lb per minute and a revolution is 2π radians, the two curves on any dyno chart always cross at 5,252 RPM. Every engine, every time. If you see a chart where they cross somewhere else, either the axes are scaled differently or the chart is wrong.

A few figures worth having in your head

Using a torque wrench without ruining the reading

Three things get this wrong more often than the number itself. Pull on the handle at the marked grip point and nowhere else, because the wrench is calibrated for that lever length and choking up on it under-torques the fastener. Pull smoothly rather than jerking, since a click-type wrench reads the peak and a snatch overshoots it. And wind a click wrench back to its lowest setting when you put it away, or the spring takes a set and the calibration drifts.

The one that surprises people is lubrication. A torque figure assumes dry, clean threads unless the manual says otherwise. Oil the threads and the same reading produces a great deal more clamping force, easily 20 to 30% more, because far less of your effort is lost to friction. That is how a correctly torqued bolt still snaps.