Hobson technical document
How to Use a Coach Screw: Pilot Hole Sizes and Installation Tips
A one-page Hobson technical article by engineer Alexander Sharp on how to install coach screws correctly. It covers the finishes and where each is used, pilot and clearance hole sizes in timber, and fixing into masonry or concrete.
What this covers
- Coach screw types and finishes
- Fixing to timber
- Pilot and clearance holes
- Fixing to masonry or concrete
Key things to remember
The essentials from this document in plain words. Check the full document before you rely on a figure.
- Coach screws are heavy duty fasteners for timber and masonry, with a hex or countersunk internal hex head, a plain shank and a tapered thread with a conical or gimlet point.
- Finish by use: zinc plated for indoors, hot dip galvanised for moderately corrosive outdoor use, 316 stainless for highly corrosive and marine sites and for treated timber.
- Always pre-drill a pilot hole so the timber does not split: half the screw diameter in softwood such as pine, three quarters in hardwood such as merbau. An M10 coach screw needs a 5mm pilot hole in softwood and 7.5mm in hardwood.
- Make the hole in the fixture 1mm bigger than the screw, to cut friction in timber to timber joints and stop metal fixings damaging the thread.
- Fit a washer under the head so it is not over-tightened and driven into the timber. Dipping the screw in grease or lanolin makes larger sizes easier to drive into hardwood.
- Into masonry or concrete, use a wall plug of the right size and follow the plug's instructions for hole size and load capacity.
Who it helps: DIY builders and tradies fixing timber with coach screws.
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This document is published by Hobson Engineering, who supply this range to us. We show it here in full so you can read it next to the product; the summary above is ours. Read it on this page or download the PDF. For the most current version, ask us.