Torque Specifications: Newton-Metres, Foot-Pounds and Why Direction Matters

Torque is rotational force: a force applied at a distance from an axis. The units are straightforward to convert and easy to apply incorrectly.

The units

• Newton-metre (N·m) — SI. One newton at one metre.
• Foot-pound (lb·ft) — 1.3558 N·m. US standard for larger fasteners.
• Inch-pound (lb·in) — 0.11298 N·m. Twelve to a foot-pound, used for small fasteners.
• Kilogram-force metre (kgf·m) — 9.80665 N·m. Older European and Japanese manuals.

Foot-pounds versus pound-feet

Strictly, torque should be written pound-feet (lb·ft), since it is a pound of force acting at a distance of feet. Foot-pounds (ft·lb) is properly a unit of energy.

They are dimensionally identical — force times distance — but physically distinct: torque is force applied perpendicular to a lever arm, energy is force applied along a displacement. In practice both spellings appear on tools and in manuals, and context makes the meaning clear.

Why torque specifications exist

A torque figure is a proxy for something you cannot measure directly: bolt preload, the tension stretching the fastener that holds the joint together.

Too little preload and the joint can loosen under vibration or separate under load. Too much and you yield or snap the bolt, or strip the threads.

The relationship between torque and preload depends heavily on friction — typically only 10–15% of your applied torque becomes tension, with the rest lost to friction under the head and in the threads. This is why lubrication matters so much: an oiled thread reaches much higher preload at the same torque, which is why applying a dry-torque specification to a lubricated bolt can overtighten it by 20–30%.

Always follow the manual's stated condition — dry, oiled, or with a specified threadlocker.

Torque wrench extensions change the reading

A frequently missed point.

Adding an extension in line with the wrench — a standard socket extension — does not change the effective lever arm, so the reading stays correct.

Adding a crow's foot or offset adapter that extends the length along the handle direction does change it, and you must correct:

wrench setting = desired torque × L / (L + E)

where L is the wrench length and E the added offset. With an 18-inch wrench, a 3-inch crow's foot and a target of 100 lb·ft, set the wrench to about 86 lb·ft.

Mount the crow's foot at 90 ° to the handle and the effective length is unchanged, so no correction is needed — a useful trick.

Sequence and staging

For multi-bolt joints such as cylinder heads or wheels, order matters as much as the value. Tightening one bolt fully distorts the joint and changes the load on its neighbours.

Standard practice is a criss-cross or star pattern, in stages: perhaps 30%, then 70%, then full torque. Wheels always use a star pattern for this reason.

Torque-to-yield bolts

Many modern engines use torque-to-yield fasteners, deliberately tightened past their elastic limit into plastic deformation. This gives very consistent preload, because a yielding bolt holds near-constant tension.

They are specified as a torque followed by an angle — "40 N·m plus 90 degrees" — and require an angle gauge. Critically, they are single-use. A stretched bolt reinstalled will not achieve correct preload and may fail.

Practical notes

• Click wrenches should be stored at their lowest setting to relax the spring.
• Calibration drifts; annual checking is sensible for anything critical.
• Never use a torque wrench to loosen fasteners.
• Pull smoothly to the click. Jerking overshoots.
• A click wrench measures only in its designed direction unless rated otherwise.

Our torque converter handles all common units, including the inch-pound and kilogram-force metre figures found in older manuals.