5/16 in x 3-1/2 in Galvanized Carriage Bolt Kit (50 Pack)

Our review of this 5/16 in x 3-1/2 in carriage bolt kit finds it aimed squarely at DIYers and contractors who need a reliable, corrosion-resistant fastener set for outdoor projects. The single biggest reason to buy is the hot dip galvanized coating combined with Grade 307A bolts and matching hardware, which delivers solid rust protection for decks, pergolas, and other exterior timber work. For anyone fastening into ACQ treated lumber or building semi-flush connections, this kit offers a balanced mix of quality and quantity in one package.

Key Features

• Hot dip galvanized coating: provides superior corrosion resistance for exterior and ACQ-treated lumber applications so fasteners last outdoors.
• Complete 50-piece kit: includes 50 carriage bolts, 50 USS thick flat washers, and 50 hex nuts so you have matching hardware for multiple assemblies without sourcing parts separately.
• Semi-flush flat head: the flat carriage head yields a semi-flush finish ideal for decks, gazebos, and pergolas where a low-profile appearance matters.
• Standard sizing: 5/16 in diameter with 3-1/2 in length and 5/16-18 nut threads fits common heavy-duty wood connections and replacement needs.
• Meets common standards: manufactured to meet or exceed relevant industry standards, which gives confidence in consistency and mechanical properties.

Who It's For

This kit is best for homeowners, carpenters, and small contractors tackling outdoor wood projects such as deck framing, rail posts, pergola beams, and gazebo assemblies who need a robust, corrosion-resistant fastener that sits semi-flush. The pack size suits jobs that require multiple identical connections or for keeping a supply on hand for repairs.

It may not be the right choice for applications requiring stainless steel or non-galvanized finishes for aesthetic or electrical reasons, or for specialty structural projects specifying higher grade nuts and bolts beyond the Grade classifications provided.

Pros & Cons

Pros

• Durable hot dip galvanized finish resists corrosion in outdoor environments.
• Well-matched hardware in one kit saves time sourcing washers and nuts separately.
• Flat carriage head gives a neat, semi-flush appearance on finished surfaces.
• Pack of 50 is practical for medium-size projects or ongoing maintenance.

Cons

• Not stainless steel, so those with the highest corrosion resistance or specific finish requirements may prefer a different alloy.
• Large pack may be more than needed for single small repairs.

Specifications

Diameter	5/16 in
Length	3-1/2 in
Head diameter	0.703 in
Washer outside diameter	7/8 in
Nut size	5/16-18, wrench size 1/2 in
Kit contents	50 Grade 307A carriage bolts, 50 USS thick washers, 50 Grade 2/A hex nuts
Coating	Hot dip galvanized for exterior use

Our Verdict

For anyone building or repairing outdoor wooden structures, this carriage bolt kit represents good value: the hot dip galvanized coating and complete matched hardware save time and reduce corrosion concerns in ACQ-treated lumber. Buy it if you need a dependable, medium-volume supply of semi-flush fasteners for decks, pergolas, or similar projects; choose stainless or a different finish only if your project requires it for appearance or extreme corrosion conditions.

Frequently Asked Questions

Are these bolts suitable for ACQ lumber?
Yes. The hot dip galvanized coating is specified as suitable for use in ACQ treated lumber.

What is included in the kit?
The kit includes 50 Grade 307A carriage bolts, 50 USS thick flat washers, and 50 Grade 2/A hex nuts.

Do the heads sit flush?
The flat carriage head provides a semi-flush finish; it is not completely flush but reduces the profile compared with round heads.