K-nut sizes.

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. An M8 K-nut is 10mm across flats where a DIN 934 nut is 13mm. M10 is 12mm against 17mm. M12 is 14mm against 19mm. That is deliberate: less metal, less weight, and it fits where a standard nut will not.

It catches people out in both directions. Reach for the 13mm because it says M8 and you will round the corners off a nut that costs a great deal more than a standard one. Order a standard nut expecting a K-nut to fit the same socket and you have bought the wrong part.

It is not a Stover, and the charts are not interchangeable

Both are one-piece all-metal locking nuts and both lock by deforming the top of the thread, so they get confused constantly. They share no dimension at all. An M8 K-nut is 10mm across flats and 7.2mm high; an M8 DIN 980 V Stover is 13mm and 8.0mm. If you have measured a nut and it is smaller than the chart says, you are probably holding a K-nut.

No DIN or ISO number — but not unstandardised

You will not find a DIN or ISO number for a metric K-nut, which catches people out when they try to specify one. They are built to German aerospace standards instead: LN9338 and LN65409 for the plain nut, LN9161 and LN65410 for the hi-temp. The captive washer version is the odd one out — its own maker calls it non-standard. The imperial cousins are MS21042 and NAS1291, which are real specifications but a different drawing, so they are not mixed in here.

Every figure on this page comes from the manufacturer's catalogue, cross-checked against their second datasheet for the same part.

We stock the zinc-nickel ones, not cadmium

The classic aerospace K-nut is cadmium plated, and cadmium is restricted under UK REACH with narrow exemptions largely for aerospace and defence. What we buy and sell is the zinc-nickel version — same 1100 MPa, same 235°C ceiling, free of cadmium and hexavalent chrome, ELV compliant, and marked with a blue dot so you can tell them apart in a bin.

For road cars, general industry and anything ELV that is the one you want anyway. If your drawing genuinely calls for cadmium plating we can get it — just say so when you order, and it is worth checking whether the specification really requires it or has simply never been revisited.

Three variants, and what each is for

The standard K-nut has its washer face built in as a ring base, so it needs no separate washer. The captive washer version swaps that for a free-spinning washer held on the nut — made for aluminium castings and other soft housings, where a bare nut would dig in, and useful anywhere a dropped washer is a nuisance. The hi-temp version is A286 stainless with silver plating and holds 650°C against 235°C, which is the one for turbo, manifold and exhaust work. The silver is not decoration: it survives the heat and stops the threads seizing when the joint comes apart.

All three lock by crimping. That locking torque drops sharply after the first refit, so where it matters, fit a new one rather than reusing.

Not sure which you need?

Bring the old one in, or send us a photo — measure across the flats and we can tell you in seconds whether it is a K-nut or a Stover. We are on Baird Close in Daventry and we stock the common metric sizes on the shelf.