I'm planning to screw some 3/4 inch lag bolts through a couple of 4x6 PT southern pine beams and looking up pilot hole diameters for softwood it seems like either 3/8" or 13/32" is recommended for the pilot hole size.

This seems fine for the threaded part of the bolts, but I'm concerned that the unthreaded part will be so thick as to split the wood (this is being done in the direction that is more likely to split).

Is this a correct pilot hole size even for lag bolts with a substantial part unthreaded?

  • 2
    If really worried, you could drill a pilot hole and then drill into that hole a less deep wider hole to fit the unthreaded diameter. Commented Sep 1, 2022 at 4:52
  • the pilot drill needs to be about 1/8 thiner then the screw portion. For the bold part without thread use same size as the bolt, but only drill as deep an neded.
    – Traveler
    Commented Sep 1, 2022 at 5:35

1 Answer 1


A "Lag Bolt" is a screw - it doesn't go all the way through (your question says through the wood). The wood needs to be step-drilled to match the diameter of the unthreaded part. A bolt with washers and nuts will need the wood to be drilled out to the diameter of the bolt.

Details here for example which gives a larger recommendation than your spec.

  • Ah, great catch on this actually being a medium wood so the pilot hole size needing to be bigger than I said. And yeah I said "through" by mistake perhaps although technically it's going through one completely and part way into another.
    – g491
    Commented Sep 1, 2022 at 4:54
  • If there will be any shear force, would it be good to have the wider portion of the step-drilled hole be slightly smaller than the shaft diameter since the wood may compress some?
    – g491
    Commented Sep 1, 2022 at 5:05

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