Machine Screw Nuts vs. Hex Nuts: Sizes, Threads and When to Use Each

A machine screw nut is a small six-sided nut made for machine screws. Fastenio's stainless steel machine screw nuts are made to ASME B18.6.3 and cover machine screw sizes from #2 through 3/8 in. A "hex nut" in most catalogs means a finished hex nut, made to a different standard (ASME B18.2.2) and sized for bolts and cap screws from 1/4 in upward. If you have a #6-32, #8-32, #10-24 or #10-32 screw, you want a machine screw nut. Fastenio doesn't carry finished hex nuts in those number sizes.

The two only overlap from 1/4 in to 3/8 in. In that range, both nuts fit the same thread, but they aren't the same part. The product pages describe the machine screw nut as thinner (lower profile) than a finished hex nut of the same thread. That means fewer threads are engaged, so use a finished hex nut when the joint carries real load. Either way, the thread has to match the screw exactly: same diameter and same threads per inch.

Is a machine screw nut the same as a hex nut?

Not quite. Both are hex nuts in shape, and both have an internal thread you match to the screw. The difference is the standard they are made to, and that standard sets the sizes and proportions:

The names cause most of the confusion. Suppliers use "hex nut," "machine screw nut" and "hex machine screw nut" loosely. When a listing doesn't name its standard, you can't be sure which proportions you're getting. Fastenio's product pages state the standard in the specifications table, so check there. Metric has no separate "machine screw nut" family: metric nuts are named by thread size and pitch, such as M8 x 1.25.

Which nut fits a #8-32 or #10-24 screw?

A machine screw nut with the same thread callout. The callout has two numbers: the screw's gauge number (which sets the diameter) and the threads per inch. A #10-24 nut fits a #10-24 screw only. It will not thread onto a #10-32, even though both are #10 diameter. The guide to matching nuts and bolts walks through checking both numbers, and the thread pitch chart lists the full UNC and UNF sizes.

Here are the common machine screw sizes, with the major diameter and TPI from Fastenio's thread pitch chart:

The two catalog columns come from the machine screw nuts catalog page and the hex nuts catalog page as of the date of this post. The catalog also lists #2-56, #3-48, #5-40 and #12-24 machine screw nuts. Product pages show current availability.

What changes when both nuts come in the same size?

At 1/4-20 and 3/8-16 you can buy either type, so the choice comes down to height and wrench size.

Height. The machine screw nut is the lower-profile part. With the same thread pitch, a shorter nut holds fewer turns of thread. That suits light-duty assembly. The product pages list electronics, instrumentation, panel and appliance work. When the joint has to be clamped hard or carries a structural load, use a full finished hex nut. Use a heavy hex nut if the design calls for one.

Wrench size. Don't assume the same wrench fits both. They're made to different standards, and Fastenio's product pages don't print the across-flats dimension for these nuts. If you're setting up tooling for a production run, measure a sample nut across the flats with calipers before you buy sockets, rather than assuming a size.

Screw length. A thinner nut leaves more of the screw exposed for the same grip. That matters when you're choosing screw length so the threads pass fully through the nut. Size the screw length for the nut you'll actually use.

Should you choose 18-8 or 316 stainless steel?

Choose by environment, not by nut type. Both nut types come in both grades. Per the catalog's material notes, 18-8 stainless steel is the standard choice for outdoor and wet environments but isn't suited to salt water or harsh chemicals. 316 stainless steel is the most corrosion-resistant stainless Fastenio carries, chosen for salt water, pool and chemical exposure. Match the nut's grade to the screw's grade. The 18-8 vs. other grades guide covers the trade-offs, and the 316 vs. 18-8 hex nut post goes into how those two grades compare for nuts specifically.

The machine screw nut range also comes in brass, silicon bronze and zinc-plated steel. Those are for jobs where the screw is the same material, such as brass hardware, marine bronze fittings or dry indoor work.

How do you pick the right nut quickly?

1. Read the screw's callout. Number size plus TPI (#10-24), or fraction plus TPI (1/4-20). If you don't know it, measure the screw and use a thread gauge.
2. Number size (#2 to #12)? Buy a machine screw nut with the identical callout.
3. 1/4 in or larger? For light assembly, either type fits. For a loaded joint, buy a finished hex nut.
4. Match the material to the screw and the environment: 18-8 for most outdoor and wet work, 316 for salt water and chemicals.
5. Check the wrench size on a sample if you're tooling up, because the two nut types are made to different standards.

The bottom line

A machine screw nut is the small, low-profile hex nut made to ASME B18.6.3 for number-size machine screws. A finished hex nut is the full-height nut made to ASME B18.2.2 for bolts from 1/4 in up. For #4 through #12 screws, buy from machine screw nuts. For corrosive or marine work, the 316 stainless steel machine screw hex nuts come in #6-32 through #12-24. For 1/4 in and up where the joint carries load, use hex nuts, such as the 316 stainless steel finished hex nuts. Pair them with matching machine screws of the same thread and grade.