What is a clearance hole?
A clearance hole is a hole that exists within a material that is large enough to allow a bolt or the thread of a screw to pass through, but small enough to prevent the screws or bolts head to pass through.
How does a clearance hole work?
When joining two pieces of material together using a screw with a fully-threaded body, the threads can push against the base (bottom) material, causing the two materials to separate rather than pulling them together. This is known as ‘jacking’.
It happens because the screw is tightening into the top material before it’s tightening into the base material.
This can create a gap between the two materials, causing a weaker joint. A clearance hole can prevent this from happening.
A clearance hole is a hole drilled in the top material to prevent jacking.
The hole should be large enough for the body of the screw to fit through, without the threads engaging with the material.
Once a clearance hole has been drilled, the screw can be inserted and driven into the base material.
The clearance hole allows the screw to tighten into the base material, pulling the two materials towards each other and clamping them in place.
When should you drill a clearance hole?
Generally, you should always drill a clearance hole when using fully-threaded screws to join tough materials (such as hardwood or metal).
Partially-threaded screws do not require a clearance hole because they have a smooth shank underneath their head, which doesn’t have any threads on. When joining two materials, the smooth shank allows the two materials to be clamped together, rather than being held apart by the threads. However, partially-threaded screws may still require a pilot hole.
Guide to drilling a clearance hole
Step 1 - Drive Screw
Drive the screw all the way through the top material.
Step 2 - Back Screw Out
Put your power drill tool in reverse and back the screw almost all the way out of the material.