Coach Screws are a big, heavy-duty screw, used for fastening wood to wood or metal to wood. The coarse thread on the screws, as seen in the image below, allows the screw to grip the material tightly.

Pre-Drilling is usually done before the coach screw application, to make sure that the wood does not split when the coach screw is tightened. This is mainly done in softwoods, or when the screw is near the edge of the wood (or the material you are drilling in to).

Pre-Drilling is also a good idea for larger coach screws. This is to make sure the coach screws go straight in smoothly, without requiring too much effort in drilling them into the material. To create the Pilot hole, use a drill bit that is slightly smaller than the diameter of the Coach Screw's shaft you are using.

For smaller sized Coach Screws, or when you are working with softwood, it isn't always necessary to pre drill into the material before the coach screw application. You may be able to drive the coach screw into the wood without pre-drilling. However, it is a good idea to test out drilling the coach screws on a spare piece of material before, so know whether or not no pre-drilling will be the best option for your project.