Pilot hole
Pilot hole: pre-drilled guide for a fastener. Sizing rules for coach screws in softwood and hardwood, and when structural screws remove the need.
Ask Chalkline about this →A pilot hole is a guide hole drilled into timber before driving a fastener, sized so the thread grips without splitting the timber or shearing the head under torque.
Also known as: pre-drill.
When it is mandatory
For coach screws, a pilot is never optional: they have no self-drilling point. Each connection needs a clearance hole through the outer member (full shank diameter) and a pilot hole into the receiving member drilled to full penetration depth.
Modern structural screws (ASSY VG, SPAX, Simpson SDWS) have self-tapping tips and drive without a pilot in softwood and most engineered timber. Pre-drilling remains required near board ends and in dense hardwoods (joint groups JD1 to JD2) per manufacturer installation guides (verified 2026-06-11, AIMS Industrial).
Sizing for coach screws
Size the pilot by timber density, not screw diameter alone. A softwood pilot runs to approximately 70% of the shank diameter; hardwood runs to approximately 80% (verified 2026-06-11, AIMS Industrial). An M10 coach screw takes a 6 mm pilot in softwood and 7 mm in hardwood. Full size-by-size tables: see coach screws.
Drill to full penetration depth, not just a starter. Under-drilling in hardwood is the most common cause of head shear on site.
Category: fixings / carpentry technique.
Related
- Coach screws for residential construction
- Structural screws for timber construction
- AS/NZS 1393: dimensional standard for metric coach screws
- Coach screw (glossary)
See also
Last updated: 2026-06-11. Verified: 2026-06-11. Quarterly review for currency.