Metallics JSFN51618 Nuts 5/16-18 Serr Flange Locknut St-zn (Jar 100)

Yes. The JSFN51618 is a 5/16 inch nut cut 18 threads per inch, so it threads onto standard 5/16-18 UNC bolts, machine screws, and threaded rod. Wrong pitch nuts in this size are rare but 5/16-24 fine thread will not start on it.

The hex across the flats measures 0.489 to 0.500 inch, so a 1/2 inch wrench or socket is the fit. Overall width across the flange runs 0.557 to 0.577 inch, which is why an open-end wrench has to seat on the hex and not ride up on the flange. Wrenching height is 0.17 inch, so a deep-well or standard socket both engage; just keep thin-wall sockets square on the hex.

The flange does the washer's job. Bearing diameter is 0.660 to 0.680 inch, which spreads load like a small flat washer and lets the nut span an oversized or sloppily aligned hole. The serrations on the underside of the flange bite into the mating surface as you torque it, so a washer under the flange would defeat the locking action. Leave it out.

That is the intended use. Metallics lists the JSFN series for high vibration areas across electrical, plumbing, and heating and air conditioning work. It takes more torque to back the nut off than it did to run it on, because the serrations have already deformed the mating surface. One caution: because the serrations gouge the material they bear on, these are not for finished or painted faces you need to keep intact, and they lose locking value if reused on the same spot.

Panel thickness is rated at 0.04 inch, roughly 20 gauge sheet. That is the minimum material the serrations can bite into without crushing or dishing the panel. On anything thinner, back it with a larger flat washer or a backing plate on the opposite face and use a plain hex nut instead, or the serrations will just tear through.

No. The JSFN51618 is flagged not Buy American compliant and not ARRA compliant, with country of origin listed as multiple. For a project with domestic-content fastener language in the spec, you need a documented domestic-source serrated flange nut instead.

Both resist back-off, by different means. The serrated flange type locks mechanically, metal into metal, so there is no temperature-sensitive insert and it clamps directly against the member. A nylon insert nut grips the bolt threads instead of the surface, so it does not mar the mating face and can be reused a few times, but the insert limits it in heat. For strut, channel, and hanger work where the face does not matter, the flange nut also removes the separate washer from your parts list, since its own bearing surface runs 0.660 to 0.680 inch.

The JSFN51618 is steel with a zinc finish, which handles indoor and sheltered damp conditions. Direct weather exposure, coastal air, or chemical washdown will eventually take the plating; for those, go to 18-8 stainless in the same serrated flange pattern. Also note that zinc plated steel bolted to stainless or aluminum sets up a dissimilar-metal couple in wet locations.

Dimensions follow ASME B18.16.4-2008, the standard for prevailing-torque metric and inch flange lock nuts, so the hex, flange diameter, and height interchange with any other nut built to that spec. Hardness is Rockwell HR15N 78 to 90. Overall nut height measures 0.268 to 0.283 inch, which matters when you are counting thread engagement on a short bolt or a strut channel nut run.

They ship in a jar of 100 pieces, and the jar is the order minimum, so 100 is the smallest quantity. The jar weighs 1.3 lbs. Metallics does offer other sizes and quantities of the serrated flange locknut on request if 5/16-18 by 100 is not the count you need for a stocking van or a shop crib.

No. This style does not gall screw threads, because the locking comes from the serrated bearing face against the joint surface rather than from deformed or pinched threads at the top of the nut. That is the practical difference from a distorted-thread (stover) type, which works the bolt threads to lock. Useful when you want to run the nut down the full length of a threaded rod without chewing it up.