How to Use a Jam Nut: Which Nut Goes First and How to Tighten It

Put the thin jam nut on first, against the part you are clamping, and the full-height nut on top of it. Tighten the jam nut down, but not to the joint's full torque, then hold it still with a wrench and tighten the full nut against it to the full torque. That is the sequence Bolt Science gives in its article on using two nuts to prevent self-loosening, and it is the same order ISO 898-2:2012 (section 5) describes, as quoted on that page: the thin nut is tightened against the parts first, then the regular or high nut is tightened against the thin nut. Fastenio's stainless steel hex jam nuts and finished hex nuts are both made to ASME B18.2.2.

The order feels backwards, which is why you see the thin nut on top so often. When the full nut is tightened down onto the thin nut, it lifts the load off the thin nut's threads and pushes the bolt thread against the opposite flanks, so the bolt is pinched between the two nuts and cannot turn relative to either one. The full nut carries the clamp load; the thin nut only holds the jam. Done the other way round, Bolt Science's vibration tests show both nuts rotating together and working loose.

How do you install a jam nut?

You need the bolt or stud, one full-height nut, one jam nut of the same thread, and two wrenches. Both nuts have to match the bolt exactly, same diameter and same threads per inch; if you're unsure how to read the thread, start with our guide to matching nuts and bolts.

1. Run the jam nut on first. Thread the thin nut down the bolt until it sits against the washer or the part.
2. Tighten the jam nut, but not all the way. Bolt Science specifies tightening the thin nut to a fraction of the joint's full torque, not the full value. The thin nut sets the preload before the threads jam; it does not carry the load afterward.
3. Run the full nut on and tighten it to the full torque. Put a second wrench on the jam nut to stop it turning, and tighten the full nut against it to the joint's torque value.
4. Stop at full torque. Bolt Science warns that overtightening the top nut keeps increasing the force between the two nuts and can strip the threads or fracture the bolt between them. Once the top nut is tight against the bottom one, the jam is formed; more torque only adds risk.

Why does the thin nut go on the bottom?

Because the bottom nut's threads end up unloaded, and the top nut's threads carry the joint. Bolt Science's explanation, in plain words: as the top nut is tightened onto the thin nut, the load comes off the thin nut's pressure flanks, and the bolt thread comes up against the opposite flanks of the thin nut. From that point the bolt thread is touching the top flanks of the bottom nut and the bottom flanks of the top nut at the same time. The threads are jammed, which is where the name comes from.

Self-loosening needs the bolt and nut threads to move relative to each other. With the threads jammed from both sides, that movement isn't available, so the nuts stay put. Since the bottom nut isn't carrying the clamp load once the jam is formed, it doesn't need full-height threads, which is why a thin nut is used there.

What happens if you put the jam nut on top?

You get something that looks locked and mostly isn't. Bolt Science ran Junker transverse vibration tests on M10 nuts and bolts in both arrangements. With the small nut on top, both nuts could be seen rotating together and then coming completely loose, a result only slightly better than a single plain nut. With the small nut next to the joint, the assembly relaxed a little but did not loosen to any significant degree. Bolt Science also notes that a thin nut tightened on top of a full nut has very little bolt extension between the two nuts to work with, so that pair is prone to loosening as the nut faces embed, even when nothing is rotating.

People put the thin nut on top for a practical reason: it's easier to get a wrench on it. With the jam nut on the bottom, the wrench holding it has to fit the gap between the part and the full nut, which usually means an open-end wrench; a socket can't reach it. With the jam nut on top you can hold the full nut with any wrench and spin the thin nut down with another. What that shortcut trades away is the jam itself. The thin nut on top is just a second nut resting on the first, and in Bolt Science's test that pair rotated together and came loose. Don't rely on it to lock a joint that vibrates.

Can you use two full nuts, or two jam nuts, instead?

Two full-height nuts, yes. Bolt Science says the two-nut method works with two standard nuts as long as the same procedure is followed: tighten the first nut partway, then hold it and tighten the second against it.

Two jam nuts stacked on each other, no. Bolt Science calls it a poor and undesirable practice when the nuts are tightened to any extent. A thin nut has a short length of thread engagement, so tightening the first thin nut can partially strip threads, often on the bolt, without anything looking wrong. Tighten the second thin nut on top and the damaged threads keep deforming after assembly, the preload drops, and the joint can come loose.

One warning from Bolt Science: on a long bolt with a lot of stretch, a thin nut at the bottom can be the wrong choice, because the bolt's extension can exceed the thread clearance inside the thin nut and strip it. Thin-nut-first is the right default, not a universal rule.

How thick is a jam nut?

Thinner than the finished hex nut of the same thread, which is the whole point. The product pages describe Fastenio's hex jam nuts as thin, low-profile nuts for use as a lock nut against a standard nut, or where clearance is tight. The exact thickness for each size is set by ASME B18.2.2, and the regular hex jam nut product pages don't print it, so check the standard's table before you design around a dimension.

The one Fastenio jam nut product that does print its height is the 18-8 stainless steel heavy hex jam nut, which has the wider across-flats of a heavy hex nut at jam-nut thickness. As listed on that page in November 2026:

The same page gives the width across flats as 1/2 in to 11/16 in across those three sizes. For comparison, the heavy hex nut product page describes the full-height heavy pattern as wider across the flats and thicker than a finished hex nut of the same thread, with more bearing area and thread engagement for high-load joints.

The sizes Fastenio carries in regular hex jam nuts, from the product pages in November 2026: 18-8 stainless steel from 1/4-20 through 1-1/2-6 in coarse and fine threads, and 316 stainless steel from 1/4-20 through 1-1/4-7. Brass and metric hex jam nuts are on the same catalog page. This post sticks to inch sizes; the procedure is the same for metric.

What else are jam nuts used for?

Not every jam nut is locking a joint. Three other jobs need no two-nut sequence:

• Setting the position of a stud. Run a jam nut onto a fully threaded stud, set the depth you want, and tighten the nut against the face of the part. The nut acts as a shoulder, so the stud stops where you set it.
• Holding an adjustment. On a leveling foot, a limit stop or a rod end, a jam nut run up against the housing holds the adjustment where you set it. Here the nut is jamming against the part, not against another nut.
• Fitting where a full nut won't. The product pages list tight clearance as the other reason to pick a jam nut: a shallow recess, a stacked assembly or a short bolt with few threads showing. In that case the jam nut is the load-carrying nut, with fewer threads engaged than a full nut, so keep it for light loads and use a finished or heavy hex nut when the joint matters.

For the broader question of jam nut versus nylon insert lock nut versus lock washer, our lock washer vs. lock nut post compares the methods. Bolt Science's own verdict is that the two-nut method, properly applied, locks better than many so-called lock nuts, but that it is time-intensive and takes some skill, which is why it's rare on new machinery.

Do stainless steel jam nuts gall?

They can, and the two-nut method gives them two chances. Stainless steel nuts and bolts gall when the threads seize under friction, and tightening one stainless nut hard against another on the same bolt is exactly the kind of high-friction, high-load contact that starts it. The precautions are the same as for any stainless fastener, and our post on preventing stainless steel galling covers them: a thin coat of anti-seize on the bolt threads, run both nuts down by hand or at low speed, no impact driver for the final tightening, and stop if a nut starts to bind before it seats. Lubricated threads also change the torque; the galling post covers that adjustment.

The bottom line

Thin nut first, against the part. Tighten it down, but not to the full torque. Full nut on top, hold the thin nut with a second wrench, tighten the full nut to the joint's full torque, and stop. That is the sequence Bolt Science documents and ISO 898-2 describes, and it is the only one of the two arrangements that held in Bolt Science's vibration tests. Thin nut on top is a shortcut that mostly gives up the lock; two thin nuts stacked is a practice to avoid.

Buy the jam nut and the full nut in the same thread: hex jam nuts in 18-8 or 316 stainless steel, matching finished hex nuts for the load-carrying nut, and heavy hex nuts where the joint needs more thread engagement.