The code for wood engineering has "lead hole" specifications for screws. They're in Chapter 12. I'll quote the text later. The pilot hole size is predicated on wood's specific gravity (basically its density—multiply by the density of water to get the wood's density), so I'll list the specific gravities of the common US softwoods first:
- Douglas Fir-Larch: G = 0.50,
- Hem-Fir: G = 0.43,
- Southern Pine: G = 0.55, and
- Spruce-Pine-Fir: G = 0.42.
Southern Pine has the same density as Mixed Maple. Hardwoods other than maple tend to have G > 0.60 (Yellow Poplar is the extreme exception). Old wood also tends to have a higher density (and therefore a higher G), as it may have been harvested from a slow growth forest instead of "farmed."
For screws loaded in withdrawal, the code mentioned earlier says:
12.1.5.2 Lead holes for wood screws loaded in withdrawal shall have a diameter equal to approximately 90% of the wood screw root diameter in wood with G > 0.6, and approximately 70% of the wood screw root diameter in wood with 0.5 < G ≤ 0.6. Wood with G ≤ 0.5 (see Table 12.3.3A) is not required to have a lead hole for insertion of wood screws.
For screws loaded laterally, the code mentioned earlier says:
12.1.5.3 Lead holes for wood screws loaded laterally shall be bored as follows:
(a) For wood with G > 0.6 (see Table 12.3.3A), the part of the lead hole receiving the shank shall have about the same diameter as the shank, and that receiving the threaded portion shall have about the same diameter as the screw at the root of the thread (see Reference 8).
(b) For G ≤ 0.6 (see Table 12.3.3A), the part of the lead hole receiving the shank shall be about 7/8 the diameter of the shank and that receiving the threaded portion shall be about 7/8 the diameter of the screw at the root of the thread (see Reference 8).
If you have a combination of lateral and withdrawal loading, then I don't know what to tell you.