Yep...Good question — this catches a lot of people out because drilling a pilot hole doesn’t always guarantee the wood won’t split.

There are a few common reasons this happens:

1. Pilot hole too small
If the pilot hole is smaller than the screw’s core (the solid part between the threads), the screw still forces the wood apart as it goes in. The pilot hole should usually match the core diameter of the screw, not the full thread diameter.

2. Wood type
Hardwoods like oak, beech, or maple are much more likely to split than softwoods. In hardwoods it’s especially important to drill the correct pilot hole size.

3. Too close to the edge of the wood
If the screw is placed near the edge of a board, there simply isn’t enough material to resist the pressure of the threads expanding the wood.

4. No clearance hole in the top piece
When joining two pieces of wood, the top piece should often have a clearance hole slightly larger than the screw threads. This allows the screw to pull the pieces together rather than forcing both pieces apart.

5. Screwing without countersinking
If the screw head pushes hard against the surface, it can cause surface splitting. A quick countersink helps prevent this.

6. Driving the screw too aggressively
High torque from a drill can force the screw in faster than the wood can compress.

A good rule is:

• Softwood: pilot hole ≈ screw core diameter

• Hardwood: pilot hole slightly larger than the core diameter

If you're unsure about sizes, a pilot hole calculator can make it much easier to choose the right drill bit for different screw sizes.

Once you get the pilot hole size right, splitting becomes much less common.

Another thing that causes splitting is driving screws too close to the edge of timber.
For hardwood you sometimes need a slightly larger pilot hole.