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Pilot Hole Diameter for Lag Bolts

Pilot or lead holes are typically used to ease the installation of large diameter lag bolts. Unless specified, the lag bolts we make will have a semi-cone point, not a gimlet point typically supplied on smaller import product. Full points are not needed for large diameter lag bolts, because pilot holes should be drilled to insure the heads do not break when torque is applied. Refer to the chart below for commonly accepted hole sizes.

There are a number of variables that may change the required pilot hole dimensions including: bolt length, diameter, and wood species. Consult an engineer for questions regarding specific applications.

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