Counterbore Hole Size for Socket Head (GB)

by The Engineer's Bible
|
Updated:
31-Oct-2022

Unsure on what size counterbore hole to use for your GB socket cap head fasteners?

Then look no further!

This short post will give you the exact dimensions you need for your screws and bolts.

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A counterbored hole is usually used for when the top of a screw or bolt needs to be flush with, or sit just beneath, the surface of the material it is inserting.

For example, a socket head cap screw with a counterbored hole sits flush with the surface, whereas the hex head screw with a simple clearance hole sits above the surface:

An example of a socket head and the corresponding counterbore hole is shown in this 3D model:

Counterbore Hole

Socket Head Cap Screw

Use the image and chart below to determine what dimensions to use, all dimensions are in millimetres.

For example, a GB M14 socket cap head bolt with a normal fit counterbored hole will require a pilot hole diameter of 15.5 mm, a counterbore diameter of 24 mm, and a counterbore depth of 15 mm.

Counterbore Hole Size Chart for Socket Cap Head Fasteners (GB)

All dimensions are in millimetres.

Fastener Size	Pilot Hole Diameter (Close Fit H12)	Pilot Hole Diameter (Normal Fit H13)	Pilot Hole Diameter (Loose Fit H14)	Counterbore Diameter	Counterbore Depth
M1.6	1.7	1.8	2.0	3.3	1.8
M2	2.2	2.4	2.6	4.3	2.3
M2.5	2.7	2.9	3.1	5.0	2.9
M3	3.2	3.4	3.6	6.0	3.4
M4	4.3	4.5	4.8	8.0	4.6
M5	5.3	5.5	5.8	10.0	5.7
M6	6.4	6.6	7.0	11.0	6.8
M8	8.4	9.0	10.0	15.0	9.0
M10	10.5	11.0	12.0	18.0	11.0
M12	13.0	13.5	14.5	20.0	13.0
Fastener Size	Pilot Hole Diameter (Close Fit H12)	Pilot Hole Diameter (Normal Fit H13)	Pilot Hole Diameter (Loose Fit H14)	Counterbore Diameter	Counterbore Depth
M14	15.0	15.5	16.5	24.0	15.0
M16	17.0	17.5	18.5	26.0	17.5
M20	21.0	22.0	24.0	33.0	21.5
M24	25.0	26.0	28.0	40.0	25.5
M30	31.0	33.0	35.0	48.0	32.0
M36	37.0	39.0	42.0	57.0	38.0