Galling is not cross-threading and it is not a manufacturing defect. It is cold welding: two metal surfaces, clean and under pressure, welding together at a microscopic level until the thread tears itself apart. Stainless is the material it happens to most, and it has a handful of very practical fixes.

Why stainless, specifically

Stainless steel protects itself with a chromium-oxide layer that re-forms instantly when it is scratched. That is exactly what makes it corrosion resistant — and it is also why it galls. When a bolt is tightened, the oxide layer is stripped from the high points of the thread, bare metal is exposed on both parts, and the two surfaces weld before a new oxide layer can form. Stainless also has a high coefficient of friction and low thermal conductivity, so the heat generated by tightening stays in the contact area and accelerates the process.

The problem gets worse with:

• Large diameters. More contact area, more heat, more torque.
• Long thread engagement. More surface in sliding contact.
• Power tools. Speed generates heat far faster than hand tightening.
• Clean, degreased, unlubricated threads. Nothing between the surfaces.
• Repeated assembly. Each cycle removes more oxide and work-hardens the surface.

Six ways to prevent it

1. Lubricate the thread. This is the single most effective measure. A nickel- or molybdenum-based anti-seize compound, or a moly-based grease, cuts the friction that generates the heat and keeps the surfaces apart. A wax-based dry film works where an oily residue is unacceptable. Note that lubrication lowers the torque needed for a given preload — reduce the torque setting accordingly, or you will over-tighten the joint.
2. Slow down. Use hand tools, or a driver on its lowest speed with a torque cut-off. If a fastener starts to feel hot, stop — the damage is already happening.
3. Torque to the correct figure, once. Over-tightening is the most common cause in practice. Work to the torque the size and grade call for, and do not “give it a bit more” as insurance.
4. Mix the alloys. A 316 (A4) nut on a 304 (A2) bolt, or a fastener with a nylon-insert nut, reduces the tendency to weld because the surfaces are not identical. Mating two identical stainless parts is the worst combination.
5. Use coarse threads and the minimum sensible engagement. A coarse thread has fewer, larger contact points and clears itself better. More thread engagement is not automatically better.
6. Do not re-use stainless fasteners indefinitely. After several assembly cycles the thread is work-hardened and the oxide has been repeatedly stripped. Treat frequently dismantled joints as consumable, or move to a lubricated or coated fastener.

If it has already seized

In order:

1. Stop turning. Continuing is what shears the shank or strips the hexagon.
2. Penetrating oil and time. Apply, leave it to creep for several hours, then try a small back-and-forth movement rather than a single hard turn.
3. Impact rather than torque. A sharp tap on the head of the fastener — not on the thread — often breaks the weld where steady force will not.
4. Local heat, carefully. Differential expansion can break the joint free, but heat will discolour stainless and can destroy a coating, and it must be kept away from anything painted, plated or polymeric.
5. Cut it off. When nothing else works, a cut-off disc is faster and cheaper than a damaged component. Then replace the fastener with a lubricated one.

What we stock for it

Stainless lines in 304 and 316, with plain, brushed and sandblasted finishes so you can pick a thread surface that suits assembly, plus nylon-insert lock nuts that avoid the stainless-on-stainless pairing entirely. Sold by the piece, no minimum order.