STANDARD COUNTERBORE DIMENSIONS SIZE CHART FOR METRIC SOCKET HEAD CAP SCREWS

Counterbore Dimensions Size Chart for recommended standard clearance and counterbore hole sizes for metric socket head cap screws from M1.6 through M48 size including M5, M6 and M8. There are two classes of fit category; close- and normal-fit category. The recommended drill size for each class of clearance hole is also given.

Metric Counterbore Size Chart:

INPUT PARAMETERS
Nominal Screw Size	M1.6 M2 M2.5 M3 M4 M5 M6 M8 M10 M12 M14 M16 M20 M24 M30 M36 M42 M48
Fit Class1,2	Normal Close

Note ¹: Close Fit: The close fit is normally limited to holes for those lengths of screws which are threaded to the head in assemblies where only one screw is to be used or where two or more screws are to be used and the mating holes are to be produced either at assembly or by matched and coordinated tooling.

Note ²: Normal Fit: The normal fit is intended for screws of relatively long length or for assemblies involving two or more screws where the mating holes are to be produced by conventional tolerancing methods.

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RESULTS
Parameter	Value
Nominal Screw Size	---	---
Max. Body Diameter [Dmax]	---	mm
Fit Class	---	--
Nominal Metric Drill Size [A]	---	mm
Counterbore Diameter [X]	---	mm
Countersink Diameter [Y] 1	---
Max. Head Height [Hmax]	---
Max. Transition Diameter [Bmax]	---

Note ¹: It is considered good practice to countersink or break the edges of holes which are smaller than (B_max) in parts having a hardness which approaches, equals or exceeds the screw hardness. If such holes are not countersunk, the heads of screws may not seat properly or the sharp edges on holes may deform the fillets on screws thereby making them susceptible to fatigue in applications involving dynamic loading. The countersink or corner relief, however, should not be larger than is necessary to insure that the fillet on the screw is cleared.

Definitions:

Counterbore: A hole used when the head of the socket head cap screw is needed to be flushed with or below the level of the workpiece's surface.

Countersink: A hole with conical shape. In this case, it's used to remove the burr left from a drilling operation thereby removing any sharp edges.

Supplements:

• Thread calculators and charts

Reference:

• Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2016) . Machinery's Handbook . 30th edition.  Industrial Press Inc.
• Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc.