Are torque specifications or values available for lag screws?

Hex Lag ScrewThere are no documented torque values for lag bolts or screws. The reason for this lies in the definition of torque when discussing threaded fasteners. To put it simply, torque is a measure of the twisting force required to spin a nut up along the threads of a bolt. Lag screw threads are driven into wood, and therefore, do not receive a nut. There are no published specifications for the amount of torque required when turning the head of a lag screw. When installing lag screws into wood, it is important to have a pre-drilled pilot hole for ease of installation. Below is a chart with recommended pilot hole diameters for a given diameter of lag screw:

Nominal Diameter of Lag Bolt, inches Shank (Unthreaded) Portion, inches Diameter of Pilot Hole, inches (Thread Portion)
38 38 1564
716 716 932
12 12 516
916 916 2364
58 58 1332
34 34 12
78 78 3964
1 1 2332
118 118 5364
114 114 1516
Table provided by OBEC Consulting Engineers

It is important to keep in mind that there are variables that could change the required pilot hole dimensions. These variables could be the lag screw’s overall length or the actual species of wood being drilled into. Although the above chart is a useful guide, it is always recommended to contact an engineer regarding any questions or concerns for a specific application.

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