Size picker · Imperial K-nut · MS21042 / MS21043
Imperial K-nut sizes.
The inch jet nut — MS21042, the one most race kit was actually designed around. Pick a size and a type for the spanner size, height and thread, and read the spanner row carefully: a 1/4-28 K-nut takes a 5/16 spanner, not the 7/16 an ordinary quarter-inch nut does.
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2 Pick a type
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Drawn to proportion from the standard dimensions. Illustration, not a photograph of the item.
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| Sym | Dimension | Value |
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Read the full nut size chart
| Size | Thread | s — across flats | m — height | dc — flange, max | c — ring base, min |
|---|---|---|---|---|---|
| UNC | 1/8" | 0.100" | 0.167" | 0.008" | |
| UNC | 5/32" | 1/8" | 0.206" | 0.010" | |
| UNC | 3/16" | 0.141" | 0.244" | 0.010" | |
| UNC | 7/32" | 0.188" | 0.290" | 0.015" | |
| UNC | 1/4" | 0.188" | 0.330" | 0.015" | |
| UNC | 5/16" | 0.219" | 0.420" | 0.019" | |
| UNC | 3/8" | 0.266" | 0.520" | 0.023" | |
| UNC | 7/16" | 0.282" | 0.620" | 0.030" | |
| UNC | 1/2" | 0.328" | 0.708" | 0.035" | |
| UNC | 9/16" | 0.410" | 0.814" | 0.040" | |
| UNC | 11/16" | 0.480" | 0.912" | 0.045" | |
| UNC | 3/4" | 0.550" | 1.014" | 0.050" |
| Size | Thread | s — across flats | m — height | dc — flange, max | c — ring base, min |
|---|---|---|---|---|---|
| UNC | 1/8" | 0.100" | 0.167" | 0.008" | |
| UNC | 5/32" | 1/8" | 0.206" | 0.010" | |
| UNC | 3/16" | 0.141" | 0.244" | 0.010" | |
| UNC | 7/32" | 0.188" | 0.290" | 0.015" | |
| UNC | 1/4" | 0.188" | 0.330" | 0.015" | |
| UNC | 5/16" | 0.219" | 0.420" | 0.019" | |
| UNC | 3/8" | 0.266" | 0.520" | 0.023" | |
| UNC | 7/16" | 0.282" | 0.620" | 0.030" | |
| UNC | 1/2" | 0.328" | 0.708" | 0.035" | |
| UNC | 9/16" | 0.410" | 0.814" | 0.040" | |
| UNC | 11/16" | 0.480" | 0.912" | 0.045" | |
| UNC | 3/4" | 0.550" | 1.014" | 0.050" |
Figures are the standards' nominal dimensions, each confirmed against two independent published tables — anything we could only source once reads "ask us" rather than printing a number we can't stand behind. Odd diameters like M7 and M9, left-hand threads and the finer pitches are all made, and availability varies by finish and class — ask us for anything you can't see.Figures are the standards' nominal dimensions, each confirmed against two independent published tables. Where only one table prints a figure, where two disagree, or where nobody publishes one at all, the row reads "ask us" rather than a number we can't stand behind. Imperial washers (BS 3410), the other BS 4320 forms, spring and shakeproof patterns and the hardness grades are all real too, and availability varies by finish — ask us for anything you can't see.
The spanner is the whole point — and the whole trap
A K-nut takes a much smaller spanner than an ordinary nut of the same thread. A 1/4-28 is 5/16 across flats against 7/16 for a B18.2.2 full nut; a 3/8-24 is 7/16 against 9/16. Less metal, less weight, and it fits where a standard nut will not — that is the design, and it is also how corners get rounded off by the wrong socket.
MS21042 is the drawing
The imperial K-nut is a genuine military standard: MS21042 (with NAS1291 alongside), self-locking, reduced hexagon, reduced height, ring base. The threads are the aerospace J-form — UNJF on the fractional sizes, UNJC on the small gauges — which take a standard UNF or UNC bolt. These are not the metric K-nuts resized: different drawing, different table, and this chart keeps them apart from the metric page on purpose.
Hi-temp means 425°C in imperial, not 650
The hi-temp imperial K-nut is MS21043: A286 stainless, silver plated, rated 425°C / 800°F. The metric hi-temp from the same maker is rated 650°C — both figures are the manufacturer's own, they genuinely differ, and mixing them up on a turbo installation matters. Dimensions are identical to the standard nut; the material and the rating are the difference you are paying for.
No captive washer in imperial
The captive-washer K-nut is a metric-only product — the maker's imperial range is hexagonal and hi-temp, full stop. If a drawing shows an imperial captive-washer nut, it is worth a call before you order.
We stock the zinc-nickel ones
The classic MS21042 plating is cadmium, which UK REACH restricts outside narrow aerospace exemptions. What we stock is the maker's zinc-nickel, cadmium-free line — same 1100 MPa, same 235°C, blue dot marking — in the common UNF sizes: 1/4-28, 5/16-24, 3/8-24 and 7/16-20 are shelf stock. The rest of the range is short lead.
All of them lock by crimping, and the locking torque drops sharply after the first refit — where it matters, fit a new one.
Not sure which you need?
Bring the old one in, or send us a photo — measure across the flats and the thread and we can name it in seconds. We are on Baird Close in Daventry.
More size charts
Every size, drawn to scale.
Hex setscrews
DIN 933 · full thread
Hex bolts
DIN 931 · part thread
Socket cap screws
DIN 912
Low head cap screws
DIN 7984
Button head screws
ISO 7380-1
Flanged button heads
ISO 7380-2
Countersunk socket screws
DIN 7991
Grub screws
DIN 913–916 · five points
Nuts
DIN 934 · nyloc · thin · Stover · flange
Washers
ISO 7089 · BS 4320 · penny
Not sure what the markings on the head mean? Bolt grades explained covers 8.8, 10.9, 12.9, stainless A2 and A4, and the imperial SAE grades.
All of it comes off the shelf at Baird Close — see the full fasteners & fixings range, or every size guide.