Counterbore and Countersink Size Chart for Socket and Flat Head Screws

I was checking a part on the bench the other day, and the socket head cap screw stood proud of the surface by a good bit. The print called for a counterbore, so the head was supposed to drop in flush, but it was sitting up like a little hat. I pulled the drawing and saw the problem right away: the counterbore depth was drawn too shallow. The bore diameter was fine, but the depth was short by about a millimeter, so the head had nowhere to go.

That tiny miss cost a rework cycle. It also reminded me how often the counterbore and countersink callouts on a print get treated as an afterthought, when they are the whole reason the fastener sits where it should. So I want to walk through the difference between the two, then hand you a counterbore and countersink size chart you can keep next to the machine. The numbers below are standard reference values, and I will tell you exactly which spec to confirm them against.

Counterbore versus countersink: the plain difference

A counterbore is a flat-bottomed recess. You drill the through hole for the screw, then you open up the top of that hole into a wider, flat cylinder. The flat bottom gives the underside of a socket head cap screw a clean seat, and the wider walls let the round head drop below the surface so it sits flush or slightly recessed. On a drawing you will see this written as CBORE or with the counterbore symbol, which looks like a small open-topped rectangle.

A countersink is a cone, not a cylinder. Instead of a flat-bottomed pocket, you cut an angled funnel into the top of the hole. That cone matches the angled underside of a flat head screw, so the head seats against the angle and ends up flush with the surface. On a drawing this shows up as CSK or with the countersink symbol, which looks like a wide shallow V.

So the quick rule is this. A socket head cap screw, which has a tall cylindrical head with a hex socket, needs a counterbore. A flat head screw, which has a tapered conical head, needs a countersink. Match the recess shape to the head shape and the part comes together the way the designer drew it.

Why the depth matters as much as the diameter

The diameter gets the head to fit inside. The depth gets the head to sit where you want it. With a counterbore, if the depth is short, the head sticks up, which is exactly what bit me on the bench. If the depth is too deep, you lose grip length and you might foul the threads or weaken the seat. Both numbers matter, and the print should give you both.

If you are unsure how tight the printed numbers really are, it is worth reading up on how to read the limits on these features. I wrote a separate piece on counterbore tolerance interpretation that covers how to treat the diameter and depth values when the drawing gives you a plus or minus.

Counterbore size chart for socket head cap screws

A counterbore for a socket head cap screw needs to clear the head diameter with a little room, and it needs enough depth to drop the head flush. The values below are standard reference dimensions for normal clearance counterbores. Confirm the exact head diameter against the fastener spec for the screw you are actually using, because head sizes vary slightly between standards and finishes.

The metric values follow the common practice tied to ISO 4762 socket head cap screws. The inch values follow common practice tied to ASME B18.3 socket head cap screws.

A few notes on how to read this table. The clearance hole is the through hole the screw shank passes through, sized for a normal fit, not a tight one. The counterbore diameter is the flat recess that clears the head. The counterbore depth is measured from the top surface down to the flat bottom, and it is set so the head sits flush; if you want the head recessed below the surface, add to the depth on purpose and note it on the print.

These are reference values for normal clearance. If your assembly needs a close fit or a loose fit, the clearance hole changes, and some shops open the counterbore depth slightly to guarantee the head clears every time. Always confirm against the controlling fastener standard before you cut metal.

Countersink size chart for flat head screws

A countersink is described by two things: the included angle of the cone and the diameter at the top surface where the cone meets the face. The angle has to match the screw head angle, or the head will not seat evenly. Get the angle wrong and the screw either sits proud or digs in on one edge.

Here is the part that trips people up. Inch flat head screws are commonly 82 degrees included angle. Metric flat head screws are commonly 90 degrees included angle. They are not interchangeable, so check which family the screw belongs to before you grab a countersink tool.

A word on how to use these countersink numbers. The countersink top diameter is the diameter of the cone where it breaks the top surface, and it is what you actually inspect, often by gaging the screw seating depth rather than the diameter alone. I have shown the top diameter a little larger than the nominal head so the head seats fully, but the exact amount depends on the screw spec and the finish, so treat the "about" values as a starting point and confirm to the fastener spec.

The metric values above follow common practice for ISO 7046 and ISO 10642 style flat head screws. The inch values follow common practice for ASME B18.6.3 flat head machine screws. Different head styles exist within each family, so always tie the number back to the exact screw on your bill of materials.

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How these callouts show up on a print

On the drawing, a counterbore line usually reads as the hole diameter, then the counterbore symbol or CBORE, then the counterbore diameter, then a depth symbol and the depth. A countersink line reads as the hole diameter, then the countersink symbol or CSK, then the top diameter and the angle. Once you have seen a few, they read quickly.

Where it gets slow is when a print uses a pile of abbreviations and stacked symbols, and you have to decode each one. If you ever get stuck on what a symbol or short code means, keep an engineering drawing abbreviations list handy so CBORE, CSK, THRU, and the rest stop slowing you down.

A quick checklist before you cut

Run through this short list before you commit to a counterbore or countersink. It catches most of the misses I have seen.

• Confirm the screw: socket head means counterbore, flat head means countersink.
• Confirm the angle for countersinks: 82 degrees for common inch flat heads, 90 degrees for common metric flat heads.
• Confirm the clearance hole size matches the fit you need, not just the screw size.
• Confirm the counterbore depth gives a flush head, and add depth on purpose if you want it recessed.
• Confirm every value against the controlling fastener spec, not just this chart.

How Mavlon handles counterbore and countersink callouts

When a drawing has a dozen holes, each with its own counterbore or countersink line, reading them by hand is slow and easy to fumble. That is the part I built Mavlon to take off your plate. Mavlon reads the drawing and pulls every dimension, tolerance, and note off the print, including the counterbore and countersink callouts and their symbols.

So when a hole line says CBORE with a diameter and a depth, or CSK with a top diameter and an angle, Mavlon picks that up and lists it for you in plain text next to the hole it belongs to. You still make the call on the spec, but you are not squinting at stacked symbols to find which hole needs which recess. It turns a slow decode into a quick review, which is exactly where a counterbore and countersink size chart and a tool like this work well together.

Frequently asked questions

What is the difference between a counterbore and a countersink?

A counterbore is a flat-bottomed recess for a socket head cap screw, which has a tall round head. A countersink is an angled cone for a flat head screw, which has a tapered head. Match the recess shape to the head shape and the fastener seats correctly.

Is a countersink 82 or 90 degrees?

It depends on the screw. Common inch flat head screws use an 82 degree included angle, while common metric flat head screws use a 90 degree included angle. Always check which family the screw belongs to before you pick the tool, because the angles are not interchangeable.

How deep should a counterbore be for a socket head cap screw?

The depth should be enough to drop the head flush with the top surface, which is roughly the head height for a normal flush fit. The standard reference depths are listed in the counterbore size chart above. If you want the head recessed below the surface, add that extra depth on purpose and note it on the print.

Where do these counterbore and countersink size chart values come from?

The values are standard reference dimensions tied to common fastener standards such as ISO 4762 and ASME B18.3 for socket head cap screws, and ISO 7046, ISO 10642, and ASME B18.6.3 for flat head screws. Head sizes vary slightly by standard and finish, so confirm against the spec for the exact screw on your part.

Can I use the same hole for both a counterbore and a countersink?

No, they are different shapes for different head styles. A flat-bottomed counterbore will not seat a conical flat head, and an angled countersink will not seat a flat-bottomed socket head. Pick the recess that matches the screw you are using.

Keep the chart close and confirm the spec

A counterbore and countersink size chart will not replace the fastener standard, but it will get you moving fast and keep you from cutting a recess that leaves the head sticking up. Print it, pin it near the machine, and confirm the final numbers to the controlling spec before you commit.

If you would rather not read every counterbore and countersink line by hand on a busy print, point Mavlon at the drawing and let it pull the callouts for you.

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