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Set Screw Dimensions and Hex Key Sizes (ISO 4026, M3 to M20)

Guide · Updated October 28, 2026

ISO 4026 covers hexagon socket set screws with flat point, the headless grub screws that hold pulleys, collars, gears and handwheels on shafts. The table gives coarse pitch, nominal hex key size s, flat point diameter dp max and min, minimum key engagement t for short and long screws, and the length at or below which a screw counts as a short size. Values agree between the fasteners.eu DIN 913 / ISO 4026 table, the RC Fastener ISO 4026 spec sheet and the Aspen Fasteners DIN 913 sheet. Check the current edition or your supplier for critical work.

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Step by step

Read the thread: Measure the major diameter and check pitch with a thread gage. This chart covers coarse pitch M3 to M20.
Pick the hex key: Set screws take a smaller key than any headed screw of the same thread: M6 is 3 mm, M8 is 4 mm, M10 is 5 mm.
Choose the point: Flat point (ISO 4026) is for frequent resetting and hard shafts. Cup point (ISO 4029) bites harder. Cone and dog points locate in a drilled spot.
Check the length: Screws at or below the short-length limit have less socket depth. Use the longer column of key engagement only on long screws.
Spot or flat the shaft: Mill a small flat or spot-drill for the point so the screw does not raise a burr that locks the hub on the shaft.
Torque and lock: Snug with a good key fully seated. Add threadlocker or a second set screw on top for vibration.

ISO 4026 set screw dimensions (mm)

Columns are: thread size, coarse pitch P, hex key s (nominal), flat point diameter dp max, dp min, key engagement t min for short screws, t min for long screws, and the length at or below which the screw is a short size (RC Fastener). Point diameters and key sizes are the same for the other ISO socket set screw points.

Size | P | s | dp max | dp min | t min short | t min long | short at or below L
M3 | 0.5 | 1.5 | 2.00 | 1.75 | 1.2 | 2 | 4
M4 | 0.7 | 2 | 2.50 | 2.25 | 1.5 | 2.5 | 5
M5 | 0.8 | 2.5 | 3.50 | 3.20 | 2 | 3 | 6
M6 | 1 | 3 | 4.00 | 3.70 | 2 | 3.5 | 8
M8 | 1.25 | 4 | 5.50 | 5.20 | 3 | 5 | 8
M10 | 1.5 | 5 | 7.00 | 6.64 | 4 | 6 | 10
M12 | 1.75 | 6 | 8.50 | 8.14 | 4.8 | 8 | 16
M16 | 2 | 8 | 12.00 | 11.57 | 6.4 | 10 | 20
M20 | 2.5 | 10 | 15.00 | 14.57 | 8 | 12 | 20

Hex key quick list

Set screw key by thread: M3 1.5 mm, M4 2 mm, M5 2.5 mm, M6 3 mm, M8 4 mm, M10 5 mm, M12 6 mm, M16 8 mm, M20 10 mm. Keep a ball-end key out of set screws; the reduced contact rounds the socket fast.

Set screw vs headed screw key sizes

M8 compared: ISO 4026 set screw 4 mm key. ISO 7380-1 button head and ISO 10642 flat head 5 mm key. ISO 4762 cap screw 6 mm key. Grabbing the cap screw key size for a set screw is the most common reason a key will not fit.

Worked example: collar on a 20 mm shaft

A shaft collar uses one M6 x 8 flat point set screw. At 8 mm long it is a short size, so expect only 2 mm minimum key engagement. Use a fresh 3 mm straight key, fully seated. Spot the shaft with a 4 mm flat (dp max 4.00 mm) so the point seats on a flat and a burr never forms where the collar slides.

Worked example: pulley that keeps walking

A pulley with an M8 flat point keeps slipping on a soft steel shaft. Swap to an M8 cup point (same 4 mm key), add a second screw at 90°, and apply medium threadlocker. If the hub still moves, add a key; set screws are not meant to carry high torque on their own.

Common problems

Socket strips: worn or ball-end key, or a short screw torqued like a long one. Use a straight key and the right length.
Hub will not slide off: point raised a burr. File the burr or put a flat on the shaft next time.
Screw loosens: vibration. Add threadlocker or a jam set screw on top.
Key too big: you are using the headed screw key. Drop one or two sizes.

FAQ

What size Allen key fits an M6 set screw?

3 mm. An M6 button or flat head takes 4 mm and a cap screw 5 mm.

Is ISO 4026 the same as DIN 913?

Yes for practical sizing. DIN 913 was replaced by ISO 4026 and the key, pitch and point diameters in this chart match both.

What does short size mean on a set screw?

Screws at or below a listed length (for example 8 mm for M6) have a shallower socket, 2 mm minimum key engagement instead of 3.5 mm, and a 120° point chamfer.

Flat point or cup point for a pulley on a shaft?

Cup point holds best on a soft or keyed shaft. Flat point is better when the hub is moved often or the shaft is hardened.

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