Pick up any bolt and three things describe it beyond its size: its thread (how the grooves are cut), its grade (how strong it is), and its finish (what’s on the outside to fight rust). None of them are complicated once you know what they’re for. Here’s each in plain terms — and when it actually matters.
Thread is just how the grooves are spaced along the bolt, counted as threads per inch. Almost every inch size comes two ways: a coarse series and a fine series. They’re not interchangeable — a coarse nut won’t spin onto a fine bolt of the same size — so it’s worth knowing which you have.
Fewer, deeper threads. The everyday default.
More, shallower threads. The precision option.
For most jobs, coarse is the right call and the easier one to live with. Reach for fine when a manufacturer calls for it, when vibration is a concern, or when you’re threading into thin material. Either way, the exact pitches available for your size are in the thread pitch & tap-drill chart.
Rule of thumb: nut and bolt must share the same thread series. If they don’t spin together freely by hand, stop — forcing it ruins both.
Grade is the bolt’s strength rating, stamped right on the head. A higher grade is a stronger bolt of the same size — same diameter, same threads, more muscle. The marks tell the story: a plain head is a Grade 2, three radial lines is a Grade 5, six lines is a Grade 8, and structural bolts carry their spec in letters like A325 or A490.
Stronger isn’t automatically better. A higher grade is harder, which can make it more brittle — and it costs more. The right move is usually to match the grade you’re replacing, and to step up only when a design or a spec calls for it. Going the other way — quietly dropping to a weaker grade — is how joints fail.
The full breakdown of head marks and what each grade can take — proof, yield, and tensile — lives in the bolt grades & strength chart.
Reading a mystery bolt? Bring it in. We’ll read the head with you and match the grade so the replacement is at least as strong as what came out.
Finish is the coating on the outside of the fastener, and its main job is corrosion resistance — keeping rust away. It mostly comes down to where the fastener lives: dry and indoors asks little, while wet, outdoor, or chemical environments ask a lot. Here are the finishes you’ll meet most:
Bare steel — the lowest cost and the least protection. Good for dry indoor use or anywhere it’ll be painted or oiled. Left out in the weather, it rusts.
A thin, shiny zinc coating — the most common finish on the shelf. Handles indoor and light outdoor exposure well. The yellow tint is mostly cosmetic; it isn’t built for constant wet or harsh weather.
A heavy, slightly rough zinc layer dipped on hot. This is the outdoor and structural workhorse — fences, framing, anything exposed for years. The coating is thick enough that the matching nuts are tapped oversize to fit.
Here the corrosion resistance is in the metal itself, not a coating — so it can’t chip or wear off. 18-8 (304) covers most needs; 316 steps up for marine and chemical exposure. Stainless trades some strength for that protection.
Engineered coatings — including the blue ceramic-filled types common in oil-field work — that add serious corrosion and sometimes anti-galling protection. When a job spec calls one out, ask us and we’ll match it.
Watch for mixing metals. Pairing plain steel with stainless, or the wrong finish in a wet spot, can speed up corrosion rather than slow it. If you’re not sure what belongs together, that’s a good counter question.
Size gets you in the neighborhood; thread, grade, and finish get you the exact right part. Keep these two charts handy — they’re the numbers behind everything above.
