Through Bolt

Bolts · thru bolt · machine bolt and nut · bolt-through fastener

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Photo: Standard production Through Bolt

Definition: A through bolt passes completely through the joint and clamps it with a nut and washers on the far side — the strongest way to bolt wood and steel where both faces are reachable.

Anatomy & Schematic

■ Head■ Shank■ Threads■ Point

Governing standards: Sized per ASME B18.2.1 (hex bolts)

Common Sizes

Inch	Metric	Head width	Head height
1/4-20 x 4	M6 x 100	7/16	5/32
3/8-16 x 6	M10 x 150	9/16	15/64
1/2-13 x 8	M12 x 200	3/4	5/16

Representative sizes. Full dimensional data lives in the standards listed below.

Grades & Strength

Grade	Material	Tensile (psi)	Marking
Grade 2	low/medium carbon steel	74,000	No head marking
Grade 5	medium carbon steel, quenched & tempered	120,000	3 radial lines
Grade 8	medium carbon alloy steel, quenched & tempered	150,000	6 radial lines

Torque Reference (in-lb)

Diameter	Grade 2	Grade 5	Grade 8
1/4-20	75	120	170
3/8-16	270	430	610
1/2-13	640	1020	1440

Dry, non-lubricated assembly values. Lubricated threads can require 20–30% less torque. Verify against your standard.

Materials

Zinc-plated steel
Hot-dip galvanized steel
Stainless steel (304 / 316)

Pros & Cons

Pros

Strongest wood-bolt joint — clamps instead of threading
Removable and re-tightenable
Full shear strength across the joint

Cons

Needs access to both sides
Requires drilling a full clearance hole
Nut and washers add cost

Typical Uses

Ledger boards and beams
Post-to-beam joints
Steel connections
Docks and playground hardware

Alternatives & Related

Lag Bolt — When only one side is reachable
Carriage Bolt — Smooth head for exposed wood surfaces
Structural Screw — High strength with no through-hole

Where to Buy

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FAQ

What is a through bolt?

A bolt that passes all the way through the parts being joined and is secured with a nut and washers — as opposed to a lag bolt, which threads into the wood itself.

What size hole do you drill for a through bolt?

A clearance hole 1/16 in larger than the bolt diameter (a 3/8 in bolt gets a 7/16 in hole). The bolt should slide through freely — the nut provides the clamp, not the hole.

Do through bolts need washers on both sides?

Yes — a washer under the head and under the nut spreads the clamp load so neither side crushes into the wood. Always use washers in wood; in steel they protect the finish and spread load.