Carriage Bolt Definition, Dimensions, Types, Grades, Torque, Uses, vs Lag Bolt

Carriage bolts are a common fastener in woodworking and construction. Their unique design and installation advantages make them a practical choice for many projects. This article covers the Carriage bolt definition, available dimensions, grades, mechanical properties, recommended torque, typical uses, and how they compare to lag bolts.

What is a Carriage Bolt?

A carriage bolt is a type of fastener commonly used in woodworking and construction projects. It features a smooth, rounded head and a flat underside, with a square neck just beneath the head. This unique design allows the bolt to be inserted through wood and prevents it from spinning as a nut is tightened on the opposite side.

The rounded top of a carriage bolt makes it less abrasive to materials like plastic, which is helpful when covering a surface that needs to remain smooth. The square section under the head bites into the wood, holding the bolt in place and stopping it from turning when tightening or loosening the nut.

Why is it Called a Carriage Bolt?

Carriage bolts get their name from their use on the frames of carriages. The wood frames were punched with square holes, which are compatible with the square shaped neck of carriage bolts. Although they are still known as carriage bolts, they’re used in various applications that require the fastening of wood to metal!

Carriage Bolt Structure

• Head: Typically, carriage bolts have a round head, although flat head styles exist. The round head provides a smooth, low-profile finish and prevents the bolt from being easily gripped and turned from above.
• Neck: Just under the head is a square or ribbed neck (depending on the design). This section plays a crucial role in installation, as it fits into a pre-drilled square hole or bites into the wood or metal to prevent the bolt from spinning as the nut is tightened.
• Shank and Threads: The shank below the neck is partially or fully threaded, depending on the bolt length and application. Unified thread pitch is most common, and the minimal thread length is typically twice the bolt diameter plus an additional fraction based on the bolt length.
• Materials and Finishes: Carriage bolts are available in various materials, including low or medium carbon steel, stainless steel (such as 18-8. 304), and are often zinc-plated, galvanized, or made from alloy steel for higher grades.

Carriage Bolt Purpose – What are Carriage Bolts Used For?

Carriage bolts are especially useful when fastening wood to metal frames or joining two pieces of wood together. They are often chosen for projects where the bolt head will be covered or where access to the head is limited. For example, when building a hoop house, carriage bolts can be used to attach wood to a metal frame, and their smooth heads help prevent damage to any plastic covering.

The square neck under the head of the carriage bolt is designed to fit snugly into a pre-drilled hole in the wood. When the nut is tightened, the square neck pulls into the wood, creating an indentation and locking the bolt in place. This design allows for secure fastening without the need to hold the bolt head with a wrench or socket, since the smooth, rounded top cannot be gripped.

Carriage bolts work well with quality plywoods such as Baltic birch or maple, and even softwoods like pine in many situations. However, materials like MDF are not recommended, as they may not hold the square neck securely enough.

Carriage Bolt Applications – Where to Use Carriage Bolts?

Carriage bolts are ideal for situations where you cannot reach the head of the bolt, such as overhead installations or places where the head will be covered. They are also favored for applications involving clamps or knobs, where the bolt needs to stay put as the clamp or knob is tightened. So carriage bolts may not always be in the spotlight, but they are essential components in countless structures and products found in daily life.

Carriage Bolt Installation – How to Use Carriage Bolts?

To install a carriage bolt, start by drilling a hole through the wood that matches the diameter of the bolt. For example, if you are using a 1/2-inch carriage bolt, drill a 1/2-inch hole. The hole needs to be just large enough for the threads to pass through, so the square neck can press into the wood and stop the bolt from spinning.

If you want the bolt head to sit below the surface of the wood, you can create a counterbored hole using a Forstner bit. This flat-bottomed hole allows the head to be recessed, making for a flush finish.

Once the hole is drilled, insert the carriage bolt and tap it gently with a hammer to seat the square neck into the wood. Place a washer and a nut on the opposite side, then tighten the nut. As you do this, the head will be pulled into the wood, holding everything securely in place. The bolt should not spin as you tighten or remove the nut, thanks to the square neck’s grip in the wood.

Carriage bolts are available in various lengths, usually in half-inch increments, so you can choose the right size for your project. If a longer bolt is used, any excess can be cut off after installation.

Carriage Bolt Grades

Bolts come in a range of grades, classes, and strength ratings. The grade of a bolt reflects its material composition, strength, and hardness. Carriage bolts are also available in several of these grades in the market, which influence their suitable applications and overall performance.

Carriage Bolt Grade Chart (Material, Tensile Strength, Hardness, Uses)

Here we’ve sorted out a table to breifly present the carriage bolt grades, each suited to specific requirements for strength, hardness, corrosion resistance, and cost.

Grade/Class	Typical Material	Tensile Strength (psi)	Hardness (HRC)	Markings/Finish	Typical Application
Grade 1 (307A)	Low/Med Carbon Steel	33,000–60,000	B Scale	Unmarked	Light-duty, non-structural
Grade 2	Low Carbon Steel	~74,000	B Scale	Unmarked	General purpose, hardware
Grade 5 (8.8)	Med Carbon Steel	120,000	25–34	3 lines/8.8, zinc	Automotive, machinery
Grade 8 (10.9)	Alloy/Med Carbon Steel	150,000	33–39	6 lines/10.9, yellow	Structural, high strength
Grade 9 (12.9)	Alloy Steel	170,000–180,000	39–44	12.9, socket cap	Heavy machinery, industrial
Bowmalloy	Alloy Steel	200,000	~45	B/Insignia	Extreme load, specialty
Stainless 18-8	304 Stainless	65,000–150,000	B Scale	None, shiny	Marine, corrosion resistance

Carriage Bolt Types

Like other bolt fasteners, carriage bolts come in different grades, with different heads, necks, and threads designed to meet different needs. The following are the most common types of carriage bolts currently on the market:

• Round Head Square Neck Carriage Bolt: The standard design, used in wood or metal.
• Short Neck Carriage Bolt: Designed for use in sheet metal where a standard neck would create an obstruction.
• Ribbed Neck Carriage Bolt: Features ribs instead of a square to resist turning in softer materials like wood or plastic.
• Fin Neck Carriage Bolt: Used in thin plywood, with two fins instead of a square or ribbed neck.
• Flat Head Square Neck Carriage Bolt: Offers a lower profile due to its flat head, commonly used in applications where a flush finish is needed.
• Knurled Carriage Bolt: Features a round head and a neck with knurled (grooved or ridged) patterns beneath the head, designed to prevent rotation when installed in wood, metal, or plastic.

Carriage Bolt Dimensions

The following sizes are involved for a standard carriage bolt’s measurement:

• Body Diameter/Thread Size: The width of threaded portion (outside thread diameter x thread pitch/TPI)
• Length: The length from the underside of the head to the end of the threaded portion.
• Head Diameter: Width of the round head.
• Head Height: Height from the base of the head to the top.
• Square Width: Width across the square neck.
• Square Depth: Height of the square neck below the head.
• Thread Length: The length of the threaded portion.

How are Carriage Bolts Measured?

When taking carriage bolt measurements the diameter and length are the main factors, they are taken in the same manner as other types of bolts and threaded fasteners. The length is taken directly from the base of the head of the bolt to the end of the threaded portion, the bolt’s diameter is the measurement of the width of the threaded portion.

So you can see the dimensions of carriage bolts are the key to selection for your project. Below we present the standard carriage bolt size charts for readers’ reference.

Metric Carriage Bolt Size Chart in MM

Dimensions in millimeters (mm). Reference: ISO 8677; DIN 603; JIS B 1171 standards.

Nominal Diameter (mm)	Pitch (mm)	Square Width Across Flat		Square Width Across Corners (Min)	Square Depth		Head Diameter		Head Height		Thread Length
		Max	Min		Min	Max	Min	Max	Min	Max	≤125	>125 & ≤200	>200
M5	0.8	5.48	4.52	5.9	4.1	2.9	13	11.7	3.1	2.5	16	20	–
M6	1	6.48	5.52	7.2	5.2	3.4	16	14.9	3.6	2.8	18	24	–
M8	1.25	8.58	7.42	9.6	6.6	4.4	20	18.7	4.8	4.2	22	28	41
M10	1.5	10.58	9.42	12.2	8.6	5.4	24	22.7	5.8	5	26	32	45
M12	1.75	12.7	11.3	14.7	10.8	6.6	30	28.7	7.6	6.2	36	38	57
M16	2	16.7	15.3	19.9	11.1	8.8	38	36.7	9.4	8.2	38	44	57
M20	2.5	20.84	19.16	24.9	15.9	14.1	46	44.4	10.9	10	52	65	65

Imperial Carriage Bolt Size Chart in Inches

All dimensions are in inches. Dimensions per ASME B18.5 1990 (R 1998) cover diameters up to 1″. Bolts over 1″ use custom head dimensions.

Bolt Diameter	Body Diameter		Head Diameter		Head Height		Square Width		Square Depth
	Max	Min	Max	Min	Max	Min	Max	Min	Max	Min
#10	0.199	0.159	0.469	0.436	0.114	0.094	0.199	0.185	0.125	0.094
1/4	0.26	0.213	0.594	0.563	0.145	0.125	0.265	0.245	0.156	0.125
5/16	0.324	0.272	0.719	0.688	0.176	0.156	0.324	0.307	0.187	0.156
3/8	0.388	0.329	0.844	0.807	0.208	0.188	0.389	0.369	0.219	0.188
7/16	0.452	0.385	0.969	0.907	0.239	0.219	0.452	0.432	0.25	0.219
1/2	0.515	0.444	1.094	1.031	0.270	0.250	0.515	0.492	0.281	0.250
5/8	0.642	0.559	1.344	1.281	0.344	0.313	0.642	0.616	0.344	0.313
3/4	0.768	0.679	1.594	1.531	0.406	0.375	0.768	0.741	0.406	0.375
7/8	0.895	0.795	1.844	1.781	0.469	0.438	0.895	0.865	0.469	0.438
1	1.022	0.91	2.094	2.031	0.531	0.500	1.022	0.990	0.531	0.500

Carriage Bolt Shear Strength Chart

The shear strength, tensile strength, and weight capacity of carriage bolts will vary by their grade, material, and diameter. For example grade 8 plain alloy steel has a shear strength of 90 thousand psi and a tensile strength of 150 thousand psi. The shear strength, tensile strength, and other limitations of a carriage bolt are important considerations for ensuring the reliability of an assembly.

Carriage Bolt Torque Chart

Here is a Carriage Bolt Torque Chart based on standard engineering practices and the mechanical properties. These values are common reference torques for carbon steel, Grade 5. Grade 8. and 18-8 stainless steel carriage bolts with zinc-plated threads lubricated.

Metric Carriage Bolt Torque Chart

Bolt Size	Class 4.6	Class 8.8	A2/A4 Stainless
M5	4 Nm (3 ft-lb)	7 Nm (5 ft-lb)	4 Nm (3 ft-lb)
M6	7 Nm (5 ft-lb)	11 Nm (8 ft-lb)	7 Nm (5 ft-lb)
M8	17 Nm (13 ft-lb)	28 Nm (21 ft-lb)	17 Nm (13 ft-lb)
M10	34 Nm (25 ft-lb)	55 Nm (41 ft-lb)	34 Nm (25 ft-lb)
M12	59 Nm (44 ft-lb)	97 Nm (72 ft-lb)	59 Nm (44 ft-lb)
M16	146 Nm (108 ft-lb)	240 Nm (177 ft-lb)	146 Nm (108 ft-lb)
M20	285 Nm (210 ft-lb)	470 Nm (347 ft-lb)	285 Nm (210 ft-lb)

Carriage Bolt Torque Chart (Imperial/SAE)

Bolt Size	Low Carbon Steel (Grade 2)*	Grade 5	Grade 8	18-8 Stainless Steel
1/4″-20	6 ft-lb (8 Nm)	9 ft-lb	12 ft-lb	6 ft-lb
5/16″-18	13 ft-lb (18 Nm)	18 ft-lb	25 ft-lb	13 ft-lb
3/8″-16	23 ft-lb (31 Nm)	31 ft-lb	44 ft-lb	23 ft-lb
7/16″-14	37 ft-lb (50 Nm)	49 ft-lb	70 ft-lb	37 ft-lb
1/2″-13	57 ft-lb (77 Nm)	80 ft-lb	120 ft-lb	57 ft-lb
5/8″-11	112 ft-lb (152 Nm)	159 ft-lb	225 ft-lb	112 ft-lb
3/4″-10	200 ft-lb (271 Nm)	276 ft-lb	400 ft-lb	200 ft-lb

How to Use This Chart

• Values assume lubricated threads. Dry threads may require higher values; consult manufacturer if unsure.
• Select the bolt size and grade that matches your application.
• Apply torque gradually: Use a calibrated torque wrench, and tighten in steps to avoid overtightening.

Note:

Always refer to manufacturer recommendations and consider application-specific factors (lubrication, material, joint type) for precise torque values.

Lag Bolt vs Carriage Bolt: What is the Difference?

What is the difference between a lag bolt and a carriage bolt? Carriage bolts and lag bolts differ in their form, although they are both applied to wood applications. Carriage bolts have a domed head and a consistent cylindrical threaded portion that ends in a flat edge. They have a square neck that prevents them from turning once they’re installed. Lag bolts have a hexagonal head wider threaded portion and pointed head.

More FAQs about Carriage Bolt

How To Install Carriage Bolts?

Carriage bolt installation starts with the bolt being inserted into a pre-drilled hole in a wood work piece, the installation hole should be compatible with the diameter of the bolt the square-shaped neck of a carriage bolt prevents it from shifting, as the nut is placed on the end and tightened which is done with a hand wrench or power tool.

How to Remove Carriage Bolts?

To remove a carriage bolt, first loosen the nut until it is near the end of the bolt but still attached. This allows you to reuse the bolt if needed. Use a hammer to tap the end of the bolt from the opposite side, pushing it back through the hole. If the bolt is stuck, you can use the claw of a hammer to pull it out. The square neck may have left an indentation in the wood, making it easier to reinstall the bolt in the same spot in the future.

Do Carriage Bolts Need Washers?

A washer is not a mandatory part of a carriage bolt assembly, because they have a wider dome shaped head. They will distribute load and are suited for placement in a softer material like wood. The square shaped collar below the bolt head will prevent rotation, thus maintaining the tension of the assembly. A washer can still be added if a material barrier is required.

Do Carriage Bolts Need Washers?

Carriage bolts can be installed without a washer, the dome-shaped round head of a carriage bolt will help to more evenly distribute load across a softer material like wood, the square shape of their collar or neck prevents the assembly from rotating and better secures it in place. If the material buffer is needed between fasteners and the installation, a washer can still be used.

What Are Carriage Bolts Made From?

Carriage bolts are most commonly manufactured from structural grade metals, which are used to make many types of bolts. Standard carriage bolts are made from carbon steel, alloy steel, and stainless steel. There sometimes finished with various treatments to enhance their resistance such as hot dipped galvanized ation and zinc plating.

Are Timber Bolts And Carriage Bolts The Same?

Timber bolts and carriage bolts are similar, but they have distinct features. Carriage bolts are used to fasten wood to metal, they have a square neck that helps to prevent turning after tightening. Timber bolts are used to connect wood to other wood parts, they also will resist turning but this is accomplished with small nubs on the underside of the head which grip the material.