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Clearance and pilot holes for wood screws

As the upper part of the shanks of wood screw are unthreaded, clearance holes should be drilled in the top piece of timber, this will allow the timber to be pulled tight onto the underlying surface. If the unthreaded shank is longer than the thickness of the top timber and the screw is larger than 6 gauge or the underlying timber is a hard wood, the clearance hole should be extended into the top of the underlying timber.

Where the underlying timber is softwood and the screw size is less than gauge 6, a drilled pilot hole is not normally required; using a bradawl to mark the position is adequate.

When using brass screws, always insert, tighten and remove a steel screw of the same size before fitting the brass screw - brass screws are relatively soft/weak and using a steel screw to cut the thread will reduce the risk of damaging the brass screw.

For metric drill sizes, see lower table.

Imperial
 
inch
Screw size
clearance
hole
pilot hole (softwood)
pilot hole (hardwood)
0
1/16
bradawl
1/32
1
5/64
bradawl
1/32
2
3/32
bradawl
3/64
3
7/64
bradawl
1/16
4
7/64
bradawl
1/16
5
1/8
bradawl
5/64
6
9/64
1/16
5/64
7
5/32
1/16
3/32
8
11/64
5/64
3/32
9
3/16
5/64
7/64
10
3/16
3/32
7/64
11
13/64
3/32
1/8
12
7/32
7/64
1/8
14
1/4
7/64
9/64
16
17/64
9/64
5/32
18
19/64
9/64
3/16
20
21/64
11/64
13/64
24
3/8
3/16
7/32

Metric
 
mm
Screw size
clearance
hole
pilot hole (softwood)
pilot hole (hardwood)
0
1.6
bradawl
0.8
1
2.0
bradawl
0.8
2
2.4
bradawl
1.2
3
2.8
bradawl
1.6
4
2.8
bradawl
1.6
5
3.2
bradawl
2.0
6
3.6
1.6
2.0
7
4.0
1.6
2.4
8
4.5
2.0
2.4
9
5.0
2.0
2.8
10
5.0
2.4
2.8
11
5.5
2.4
3.2
12
5.5
2.8
3.2
14
6.5
2.8
3.6
16
7.0
3.6
4.0
18
7.5
3.6
5.0
20
8.5
4.5
5.5
24
9.0
5.0
5.5

 

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