Three words you’ll hear at the counter — and what each one actually changes about the fastener in your hand.
Beyond its size, three things describe any bolt: its thread, its grade, and its finish. Here’s where each one lives on the bolt:
Thread — coarse vs. fine
Thread is how tightly the grooves are packed, counted as threads per inch. Most inch sizes come two ways. Same bolt size, one inch of each:
The everyday default
- Faster to run on and off
- Shrugs off nicks, dirt, and plating
- Strips less in soft material
The precision option
- A bit stronger at the same size
- Holds better under vibration
- Automotive, aerospace, machinery
Not sure? Coarse is usually right. Use fine when the maker calls for it. Every size’s pitches are in the thread pitch & tap-drill chart.
Rule of thumb: nut and bolt must share the same thread. If they don’t spin together by hand, stop — forcing it ruins both.
Grade — how strong
Grade is the bolt’s strength rating, stamped on top of the head. Look down at the head and count the lines:
Bars show minimum tensile strength (how hard you can pull before it breaks), common sizes. Every head mark and every number by size is in the bolt grades & strength chart.
Replacing a Grade 5? Here’s the rule:
Mystery bolt? Bring it in. We’ll read the head with you and match the grade.
Finish — fighting rust
Finish is the coating on the outside, and its job is keeping rust away. Pick it by where the fastener will live:
Dry & indoors
Plain or zinc plated
Outdoors, in the weather
Hot-dip galvanized or 18-8 stainless
Salt, pools, chemicals
316 stainless or a specialty coating
Bare steel. Cheapest, least protection. Fine dry and indoors, or where it gets painted or oiled. Rusts outside.
A black chemical finish that adds no thickness. On its own it does little against rust; the light oil it usually gets helps a bit. Chosen mostly for the look — indoors and dry.
Thin, shiny zinc — the most common finish on the shelf. Indoor and light outdoor. Yellow is mostly cosmetic; not for constant wet.
A hard, bright silver plating that wears well and looks good. More rust protection than zinc. Common on decorative and electrical parts.
Silver-gray plating with strong rust protection, long used on aircraft and marine parts. Cadmium is toxic, so its use is restricted today.
Heavy, dull, slightly rough. The outdoor and structural workhorse. The coat is so thick the matching nuts are tapped oversize — use galvanized nuts.
No coating to chip off — the rust resistance is in the steel. Covers most outdoor needs. Trades some strength for it.
Stainless that can be hardened — stronger than 18-8, and magnetic, which is why self-drilling screws use it. Less rust resistance than 18-8: fine in mild weather, not for salt or chemicals.
The step up from 18-8 for salt water, pools, and chemicals.
The long-time standard for salt water and many chemicals — boats, docks, chemical plants. Strong and very corrosion resistant, and it costs more.
Won’t rust, carries electricity, and looks good — electrical, plumbing, and decorative work. Softer than steel; not for heavy loads.
Much lighter than steel and won’t rust, but softer. Best with aluminum parts: against steel or stainless in the wet, the aluminum corrodes.
Engineered coatings, like the blue ones common in oil-field work. When a job spec names one, ask us and we’ll match it.
Watch for mixing metals. Plain steel touching stainless, or the wrong finish in a wet spot, can make rust faster. Not sure what goes together? Ask at the counter.
Put it together
Size gets you close. Thread, grade, and finish get you the exact part. These are the numbers behind everything above: