How to Calculate the Correct Torque Settings for an AR
Short Story: If you are adding any appreciable length to the lever arm of your torque wrench, you should adjust your setting accordingly.
Torque specifications don’t change. If the barrel nut calls for 55 ft. lbs., that’s how much force you need to apply.
Torque wrenches are calibrated based on the length of the wrench arm from the center of the square driver. When you add any substantial length to the wrench via the attachment, you are changing the actual torque at a given setting.
When you use our recommended wrench attachments, you can easily use the wrench at 90 degrees and not need to adjust the setting. If you use the attachment in line with the wrench handle (or don’t have the choice, as with armorer’s wrenches), you should calculate and use an adjusted value to account for the added length.
The diagram below illustrates the effect that the alignment of the axes has on the torque arm length:
How to Calculated Adjusted Torque Setting
Identify your target torque. This is T1.
Measure the length of the wrench from the center of the square driver to the center of the grip. This is L1.
Next, measure the length from the center of the square socket on the attachment to the center of the attachment axis (center of whatever you are torqueing). This is L2.
To calculate your adjusted torque setting (T2):
The impact of adding an inch to a 18″ wrench is minimal; you don’t have to worry much. You should adjust your setting to 52 ft. lbs. (55 * 18″ / 19″). If you dial 55 ft. lbs. into your wrench, you are actually applying 58 ft. lbs. of torque; 5.6% (1″ ÷ 18″) more than the nominal/dialed value, but probably not a huge deal.
Now, if you are adding 8″ with an AR-15 armorer’s wrench, that’s a different story. You should adjust your setting to 38 ft. lbs. (55 * 18″ / 26″). Without this adjustment, that additional leverage is amplifying the nominal/dialed 55 ft. lbs. of torque to nearly 80 ft. lbs.; 44.4% (8″ ÷ 18″) more than the nominal/dialed value. That could mean stripped threads.