Metallics JMTLN4SS Nut M4 Hex Kep Lock Nut 18-8 SS (Jar 100)

It is an M4 nut, so it threads onto M4 metric machine screws and bolts (M4 x 0.7 coarse pitch is the standard for M4 fasteners). It will not run onto a #8-32 or any other inch-series screw, so if your enclosure hardware is inch rather than metric you need a different nut.

A kep nut (also called a K-lock) is a hex nut with a free-spinning external-tooth lock washer captured under the flange. The teeth bite into the mounting surface as you tighten, which resists loosening from vibration, and because the washer is already attached you do not add a separate star washer. One piece instead of two on the assembly line, which is why these show up on panel and enclosure work.

The nut is 18-8 stainless steel, the common 300-series austenitic grade, so it holds up in damp indoor and general outdoor exposure far better than a plated-steel kep nut. 18-8 is not the grade to pick for direct saltwater or chloride-heavy chemical exposure, where 316 is the usual step up. One thing to watch: stainless on stainless galls, so run it on without hammering it down dry.

They ship as a jar of 100 pieces, which is the standard small-fastener pack for this line. Handy for a van or a shop drawer rather than a bulk box, so if you need several hundred for a panel build you are ordering multiple jars.

That is exactly the trade it is made for. A plain hex nut plus a separate split or star washer does the same job but is two loose parts to handle; the kep nut carries its toothed washer already assembled, so you get the vibration resistance with one part. Where you want a reusable prevailing-torque lock instead, a nylon insert lock nut is the other direction to go, though nylon insert nuts are temperature limited where a steel-toothed kep nut is not.

Order it as Metallics JMTLN4SS. That number covers the M4 hex kep lock nut in 18-8 stainless, supplied in a 100-piece jar, and there is only one configuration of it, so there is no finish or pack option to specify.