Bolts, Nuts and Anchor Bolts: Grades, Sizes and Uses

A bolt is the cheapest part of a steel connection and the first part to fail when it is wrong. Two bolts can look identical while one is ordinary mild steel and the other a high-tension bolt twice as strong — the difference is a grade number stamped on the head.

Decode the grade numbers

• Grade 4.6 (mild steel): 400 MPa ultimate strength, 240 MPa yield. The general-purpose bolt for gates, brackets, fencing and light fabrication.
• Grade 8.8 (high tension): 800 MPa ultimate, 640 MPa yield. The standard structural bolt — trusses, purlin connections, machinery bases.
• Grade 10.9: 1000 MPa ultimate. High-stress joints, automotive and heavy machinery work.

The rule: never mix grades in one joint. A grade 4.6 nut on an 8.8 bolt strips long before the bolt reaches its load — pair nuts with bolts of the same grade.

Common sizes and what they carry

• M6 – M8: brackets, cable trays, light sheet work
• M10 – M12: purlins, fence posts, general fabrication — the site workhorse sizes
• M16 – M20: beam-to-beam connections, equipment bases
• M24 and up: structural column bases, heavy plant

As a rule of thumb, thread engagement of about 1.5 times the bolt diameter gives a full-strength joint — an M12 bolt wants an 18–20 mm thick nut or threaded boss.

Anchor bolts: fixing steel to concrete

• J bolts and L bolts are cast into the concrete while it is poured — the classic fixing for column base plates and machinery foundations. The bent tail grips the concrete so the bolt cannot pull out.
• Wedge and chemical anchors are drilled in after the concrete has cured, for retrofits and repairs where nothing was cast in.

For base plates, get the projection right before the pour: the thread should stand proud enough for the plate, two nuts and a washer — roughly bolt diameter plus 40–60 mm.

Coating decides outdoor life

Plain black bolts rust quickly outside — fine for indoor fabrication, wrong for anything weather-exposed. Galvanized bolts carry a zinc layer that sacrifices itself before the steel rusts, which is what roofing, fencing and coastal work demands. Galvanized pairs with galvanized nuts (plain nuts on galvanized bolts start rusting at the thread).

FAQ

Can I reuse high-tension bolts? In structural joints, no — 8.8 and 10.9 bolts are sized for one controlled tightening. For gates and brackets, reuse is fine as long as threads are undamaged.

Why is my galvanized bolt stiff in the nut? Zinc builds up on threads during galvanizing. Run a die or thread-chaser over it once, and add a little grease — the joint is not damaged.

Seegaan stocks high tension bolt nuts in all sizes, J bolts, L hooks and anchor bolts from Colombo, delivered island-wide. Odd sizes and bulk project quantities are our daily work — call +94 11 239 6384 or send the list through the quote page.

AI-generated (Flux via Writeseed), 2026-11-10