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Fine Thread Nylon Insert Flange Lock Nut Dimensions (ISO 12125, M8x1 to M20x1.5)

Guide · Updated October 28, 2026

ISO 12125 covers prevailing torque hexagon nuts with flange and a non-metallic (nylon) insert, 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; the nylon ring adds height. Values come from the ISO 12125:2012 preview table, checked against the Engineering Hardware ISO 12125 table (every column, every size) and our ISO 10663 chart (s, e, dc, dw, thread height and mw). Height h max also matches the coarse thread ISO 7043 table from Wanhong. Not shown: h min. ISO 12125 lists low h min values (M8 8.74 mm) while the coarse ISO 7043 tables list 10.4 mm, so we left h min out until a third source confirms it. Also not shown: da, c and r. Note: ISO 12126 is the all-metal version, not the nylon one.

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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 nylon ring: M12 16.10 mm, M16 20.30 mm. Use h max, not thread height, for bolt length math.
Leave thread past the insert: Run at least 2 full threads past the top of the nylon so the insert grips.
Do not reuse hot or worn nuts: Nylon inserts lose grip after heat (above about 120 °C) or many reuses. Use an all-metal ISO 12126 nut for hot spots.

ISO 12125 fine thread nylon insert 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 (includes nylon insert), 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.

Size | s max | s min | h max | m min | mw min | e min | dc max | dw min
M8x1 | 13.00 | 12.73 | 11.10 | 7.64 | 4.6 | 14.38 | 17.9 | 15.8
M10x1 / (M10x1.25) | 15.00 | 14.73 | 13.50 | 9.64 | 5.6 | 16.64 | 21.8 | 19.6
M12x1.5 / (M12x1.25) | 18.00 | 17.73 | 16.10 | 11.57 | 6.8 | 20.03 | 26.0 | 23.8
(M14x1.5) | 21.00 | 20.67 | 18.20 | 13.30 | 7.7 | 23.36 | 29.9 | 27.6
M16x1.5 | 24.00 | 23.67 | 20.30 | 15.30 | 8.9 | 26.75 | 34.5 | 31.9
M20x1.5 | 30.00 | 29.16 | 24.80 | 18.70 | 10.7 | 32.95 | 42.8 | 39.9

Nylon vs all-metal 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 12125 nylon insert 16.10 mm, ISO 12126 all-metal 13.80 mm, ISO 10663 plain 12.00 mm. Look for the nylon ring or measure height to tell them apart.

Worked example: bolt length for an M12x1.5 nylon flange nut

Grip 20 mm, nut h max 16.10 mm, plus 2 threads at 1.5 mm = 3.0 mm. Total 39.1 mm, so an M12x1.5 x 40 works.

Common problems

Coarse bolt, fine nut: it starts, then binds. Check pitch first.
Bolt too short: the end stops inside the nylon and the nut does not lock. Add length.
Nylon nut near an exhaust or heater: the insert softens. Use ISO 12126 all-metal.

FAQ

What is the difference between ISO 12125 and ISO 12126?

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

What is the difference between ISO 12125 and ISO 10663?

ISO 10663 is the plain fine pitch flange nut. ISO 12125 has the same hex, flange and bearing face plus a nylon locking insert, so it is taller.

What is the difference between ISO 12125 and ISO 7043?

ISO 7043 is the coarse thread nylon insert flange nut. Hex, flange and h max are the same; only the thread pitch differs.

What wrench fits an M10x1 ISO 12125 nut?

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

How tall is an M16x1.5 nylon flange lock nut?

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

Which pitches are preferred?

ISO 12125 prefers M10x1 and M12x1.5. M10x1.25, M12x1.25 and M14x1.5 are listed in parentheses: avoid them if possible.

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General shop information only — follow your drawing, tool maker's data and your quality system.