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Fine Thread All-Metal Flange Lock Nut Dimensions (ISO 12126, M8x1 to M20x1.5)

Guide · Updated October 28, 2026

ISO 12126 covers prevailing torque all-metal hexagon nuts with flange, style 2, with metric fine pitch thread from M8x1 to M20x1.5, grade A through M16 and grade B for M20, property classes 6, 8 and 10. Hex, flange and bearing face match ISO 10663 plus a prevailing torque feature. Values come from the ISO 12126:2012 preview table, checked against the Engineering Hardware ISO 12126 table and the GlobalFastener ISO 12126 table (every column, every size), and against our ISO 10663 chart for s, e, dc, dw, thread height and mw. One conflict: for M16 the ISO 12126 preview and the two tables copied from it list e min 26.00 mm, while ISO 10663, ISO 12125 and ISO 4161 list 26.75 mm for the same 24 mm hex. We left that cell out. Not shown: h min, da, c and r. Note: ISO 12125 is the nylon-insert version.

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Step by step

Confirm fine pitch: Check pitch with a thread gage. An M12x1.5 lock nut will not run on a coarse M12x1.75 bolt, and M12x1.25 is not M12x1.5.
Pick the wrench: Same hex as ISO 10663: M8 13 mm, M10 15 mm, M12 18 mm, M14 21 mm, M16 24 mm, M20 30 mm.
Check the flange: The flange dc max is the clearance you need around the hole: M12 26.0 mm, M16 34.5 mm. No washer is needed.
Use total height for length: h max includes the locking top: M12 13.80 mm, M16 18.30 mm. Use h max for bolt length math.
Leave thread past the nut: Run at least 2 full threads past the top so the distorted locking threads fully engage.
Use all-metal where it is hot: No nylon to soften, so ISO 12126 suits exhaust, engine and other hot spots where a nylon ISO 12125 nut would lose grip.

ISO 12126 fine thread all-metal flange lock nut dimensions (mm)

Columns: thread size x pitch, width across flats s max and min (s max is the wrench size), total height h max, thread height m min, wrenching height mw min, width across corners e min, flange diameter dc max, bearing face diameter dw min. Sizes in parentheses should be avoided if possible. M16 e min: see note above.

Size | s max | s min | h max | m min | mw min | e min | dc max | dw min
M8x1 | 13.00 | 12.73 | 9.40 | 7.64 | 4.6 | 14.38 | 17.9 | 15.8
M10x1 / (M10x1.25) | 15.00 | 14.73 | 11.40 | 9.64 | 5.6 | 16.64 | 21.8 | 19.6
M12x1.5 / (M12x1.25) | 18.00 | 17.73 | 13.80 | 11.57 | 6.8 | 20.03 | 26.0 | 23.8
(M14x1.5) | 21.00 | 20.67 | 15.90 | 13.30 | 7.7 | 23.36 | 29.9 | 27.6
M16x1.5 | 24.00 | 23.67 | 18.30 | 15.30 | 8.9 | — (conflict) | 34.5 | 31.9
M20x1.5 | 30.00 | 29.16 | 22.40 | 18.70 | 10.7 | 32.95 | 42.8 | 39.9

All-metal vs nylon vs plain fine flange nut

For M12x1.5 all three take an 18 mm wrench and have a 26.0 mm flange. Height max: ISO 12126 all-metal 13.80 mm, ISO 12125 nylon insert 16.10 mm, ISO 10663 plain 12.00 mm. No visible nylon ring and an out-of-round or crimped top means all-metal.

Worked example: bolt length for an M12x1.5 all-metal flange nut

Grip 20 mm, nut h max 13.80 mm, plus 2 threads at 1.5 mm = 3.0 mm. Total 36.8 mm, so an M12x1.5 x 40 works (M12x1.5 x 35 is too short).

Common problems

Coarse bolt, fine nut: it starts, then binds. Check pitch first.
Galling on stainless: all-metal lock nuts can seize on stainless bolts. Lubricate and run them on slowly.
Reuse: prevailing torque drops each time. Replace nuts on critical joints.

FAQ

What is the difference between ISO 12126 and ISO 12125?

Both are fine pitch hex flange lock nuts with the same hex and flange. ISO 12126 is all-metal (M12 h max 13.80 mm). ISO 12125 has a nylon insert and is taller (M12 h max 16.10 mm).

What is the difference between ISO 12126 and ISO 10663?

ISO 10663 is the plain fine pitch flange nut. ISO 12126 has the same hex, flange and bearing face plus an all-metal prevailing torque feature, so it is slightly taller.

What is the difference between ISO 12126 and ISO 7042?

ISO 7042 is a coarse thread all-metal lock nut without a flange. ISO 12126 is fine pitch and has a built-in flange.

What wrench fits an M10x1 ISO 12126 nut?

15 mm. ISO flange nuts use 15 mm at M10, not 16 or 17 mm.

How tall is an M16x1.5 all-metal flange lock nut?

18.30 mm max overall, with at least 15.30 mm of thread height.

Why is M16 e min not shown?

The ISO 12126:2012 preview lists 26.00 mm, but ISO 10663, ISO 12125 and ISO 4161 (same hex) list 26.75 mm, which also matches 1.13 x s min. Until this is settled, use 26.75 mm from ISO 10663 as a guide and check the current standard.

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