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Torque wrench extension calculator (crowfoot and adapter)

When a crowfoot or offset adapter moves the fastener away from the wrench's square drive, the wrench applies more torque than it reads. Set it to T × L ÷ (L + E): for 100 ft-lb with an 18 in wrench and a 2 in adapter in line, set 90.

By the Torque Reference editors 4 sources cited Last reviewed Method
Contents7 sections

in

in

90.0 to set on the wrench100 × 18 ÷ (18 + 2) = 90.0, in the same unit as the required torque, with the extension in line with the wrench.

Key takeaways

  • Wrench setting = required torque × L ÷ (L + E), where L is handle grip to drive center and E is drive center to fastener center ⁠1.
  • A straight extension bar in line with the drive does not change the torque. Only an offset does ⁠1.
  • An adapter turned 90° to the wrench needs no correction, because it adds no length along the wrench ⁠1.
  • Leaving the wrench at the spec with an offset adapter over-tightens: a 3 in adapter on an 18.5 in wrench adds 16%.

Quick facts

FormulaT_set = T × L ÷ (L + E)
LCenter of handle grip to drive center
EDrive center to fastener center, along the wrench
UnitsAny, as long as L and E match

How it works

A torque wrench measures the moment about its own square drive. Hang a crowfoot or adapter off that drive and the fastener sits further from your hand, so the same pull makes more torque at the fastener. Terex’s service tech tip gives the correction as (torque wrench length × torque desired) ÷ (torque wrench length + extension length) ⁠1. Written as a formula:

T_set = T × L ÷ (L + E)

  • T is the torque the fastener needs.
  • L is the wrench length from the drive center to the center of the handle. Some wrenches have a mark on the handle. If yours doesn’t, mark the middle of the grip ⁠1. Norbar notes that calibration loads the wrench at the center of the handle or at the maker’s marked load point ⁠2, so that is where your hand should be too.
  • E is the distance from the wrench drive center to the adapter drive center, measured center to center ⁠1.

The answer comes out in the same unit as T. L and E just need to share a unit with each other. The calculator accepts a negative E for an adapter that points back toward the handle, which makes the setting higher than the spec.

Worked example

The calculator default: 100 ft-lb needed, 18 in wrench, 2 in adapter in line.

T_set = 100 × 18 ÷ (18 + 2) = 90.0 ft-lb

Terex works the same formula with a 180 ft-lb spec, an 18.5 in wrench and a 3 in adapter: 180 × 18.5 ÷ 21.5 = 155 ft-lb ⁠1.

Angle changes the effective E:

180 ft-lb spec, 18.5 in wrench, 3 in adapter at different angles ⁠1
Adapter position Effective E Set the wrench to
In line with the wrench 3 in 155 ft-lb
45° to the wrench 2.125 in (measured) 161 ft-lb
90° to the wrench 0 180 ft-lb

At 90° the adapter’s length is square to the handle, so the fastener sits the same distance from your hand as the drive does and no correction is needed ⁠1.

Reading the result

The setting is what you dial on a click wrench or read on a dial or digital wrench. When it clicks, the fastener has the torque you wanted, assuming the wrench is in calibration and you pulled at the grip.

If you skip the correction, the fastener gets T_set × (L + E) ÷ L. Setting 180 on the 18.5 in wrench with the 3 in adapter in line delivers 180 × 21.5 ÷ 18.5 = 209 ft-lb, 16% over. Short wrenches suffer more. The same 3 in adapter on a 12 in wrench adds 25%.

Two accessories don’t need the formula. A straight extension bar between the wrench and the socket adds no offset ⁠1. A universal joint or swivel socket doesn’t change the applied torque at angles of 15° or less ⁠1.

Angle adapters are a different tool

A torque-angle gauge or angle adapter measures rotation, not torque. It is for torque-plus-angle procedures, mostly torque-to-yield bolts. Subaru’s head bolt procedure for the STI requires both a quality torque wrench and a torque angle gauge, with a calibration check if either has not been calibrated in the prior 6 months ⁠3. FCA’s Pentastar head bolt steps say “Do not use a torque wrench for this step” for the angle stages ⁠4. There is nothing to correct with this calculator for an angle step: an adapter changes torque, but a 90° turn is a 90° turn. See torque-to-yield bolts.

Common mistakes

  • Measuring L to the end of the handle. Use the center of the grip or the marked load point, not the tip.
  • Measuring E to the edge of the crowfoot. E runs center to center: wrench drive to fastener.
  • Correcting a 90° adapter. Square to the wrench, E is zero.
  • Forgetting a negative offset. An adapter pointing back toward the handle shortens the lever, so the setting must be higher than the spec. Enter E as a negative number.
  • Mixing units. Feet for L and inches for E gives nonsense. The torque unit, by contrast, can be anything: the formula is a ratio.

For unit changes, use the torque converter.

Sources

  1. 1
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  4. 4

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