Are torque specifications or values available for lag screws?
There 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) |
|---|---|---|
| 3⁄8 | 3⁄8 | 15⁄64 |
| 7⁄16 | 7⁄16 | 9⁄32 |
| 1⁄2 | 1⁄2 | 5⁄16 |
| 9⁄16 | 9⁄16 | 23⁄64 |
| 5⁄8 | 5⁄8 | 13⁄32 |
| 3⁄4 | 3⁄4 | 1⁄2 |
| 7⁄8 | 7⁄8 | 39⁄64 |
| 1 | 1 | 23⁄32 |
| 11⁄8 | 11⁄8 | 53⁄64 |
| 11⁄4 | 11⁄4 | 15⁄16 |
| 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.
Even if torque value for installation are not available for the good reasons you mention, there should be safe max value of torque that does not cause excessive fatigue an can lead to a broken bolt.
@Steve- We agree that would be ideal, but we have yet to locate any reliable data.
The spec is not for the bolt, but for the tool , impact driver, that is spinning the bolt to attach a PT 2×10 to a PT 6×6 post,
Does Portland produce a stainless steel torque washer for carriage bolts?
@Dave – I am sorry, we do not have such an item.