Pieces of wood, wood to metal, and metal to metal are all secured with carriage bolts. They belong to the round head bolt family and have a round head with a low rounded top surface and flat bearing surface and an integrally formed square neck under the head.
Carriage bolts are also known as carriage screws, round head square neck bolts, coach bolts, and shaker screen bolts and are commonly found on chain-link fences and outdoor wood furniture, and swing sets.
Typically, they’re tightened by torquing a nut since the head isn’t designed to turn. It is available for use with an internally threaded or tapped hole and tightened by turning the carriage bolt’s square neck.
In our guide, you can find out what is a carriage bolt used for, how to use carriage bolts, and what size carriage bolt to use. (Find the Best Air Ratchet)
By the end, you’ll understand how to read our carriage bolt sizes chart to get the right information on carriage bolt lengths and size.
What Are the Sizes of Carriage Bolts?
Standard right-hand and Unified inch coarse series threads are used (UNC, Unified National Coarse). 6″ and shorter lengths are normally fully threaded, while lengths longer than 6″ are frequently halfway threaded.
Carriage bolts are typically available in diameters ranging from #10 to 3/4″ and lengths ranging from about 1/2″ to 20″—only larger sizes are available in longer lengths.
The head diameter is usually double the size of the bolt or somewhat more for smaller sizes. The square neck’s width across flats is about the same as the bolt’s size. The head diameter and height and the square neck width and depth are all visible here.
The most common types of carriage bolts are grades 2 and 5, steel, and stainless steel.
As the nut is tightened, the square neck stops the bolt from turning. As a result, there is no need for access to the head, and it can be fitted in a counterbore flat bottom recess if desired. When working with softwood, one source recommends drilling the hole the same size as the carriage bolt’s shank to ensure a tight fit for the square neck; drive the bolt into position with a “soft” hammer or mallet to avoid damaging the finish.
A minor counterbore for the neck is occasionally needed to prevent the wood from splitting and ensure that the head pulls down flush in hardwood. To accommodate the square neck, which restricts bolt rotation, metal elements like chain-link fence tension and brace bands are commonly square pierced. (Read Craftsman Socket Sizes Chart)
Bolts, washers, and nuts should be of similar materials and finishes. Always use hot-dipped galvanized nuts with hot-dipped galvanized bolts because they are tapped oversize (threaded larger than typical) to accommodate the thick zinc coating on the bolts.
When you use a galvanized nut on a non-galvanized bolt, it leads to a loose fit, and using a plain finish or zinc plated nut on a hot-dip galvanized bolt will cause a much too much fit tight).
Although fin and ribbed neck carriage bolts are available, square neck and, occasionally, short neck carriage bolts are the most common.
Here are the carriage bolt lengths:
|Carriage Bolt Dimensions|
|Nominal Size||Head Diameter||Head Height||Square Width||Square Depth||Square Depth|
How Are Carriage Bolts Measured?
Carriage bolt lengths are calculated from the expected material surface to the end of the fastener. The measurement for fasteners with heads that sit above the surface is taken from directly under the head to the end of the fastener.
The measurement for countersunk fasteners is taken from the point on the head where the material’s surface is to the end of the fastener.
Here is how to measure machine bolts vs. carriage bolts. Hex bolts are measured from under the head to the tip of the bolt.
Carriage bolts dimensions are measured from under the head and the length of the shank.
Sheet Metal Screws
Sheet metal screw measurements vary on the type of screw you have. Their overall length in inches measures countersunk screws being flat or oval heads.
If you have any pan Head, truss head, or hex head screws, you measure the thread from under the fastener head. (Read What Size Sockets Do I Need)
Lag Bolts or lag screws are measured from under the head.
Wood Screws vary based on the type of wood screw you are using. You can have the countersunk, flat, or oval head that ate measured overall.
If you have round head screws, these are measured from under the head, including the short neck that may have no fastening thread.
Machine screw measurement depends on the style of the head it has. If you have countersunk with Flat or oval head, you measure the overall length of your screws are pan-head or round head, you take your measurement from under the head.
Socket screw measurements vary on the type of screw you have. The overall length will measure flat Socket Caps. Whereas socket head caps and button head caps will be measured under their head.
- Eye Bolts: Eye Bolts get measures from under the eye.
- Hanger Bolts: Hanger bolt length will measure from end to end.
- Shoulder Bolts: Shoulder bolt length gets a measurement taken from the length of the shoulder.
- Sex Bolts: Sex bolt length will be from under the head.
What size is the square on a carriage bolt?
Dimensions per ASME B18.5 cover a diameter up to 1″. Bolts over 1″ use custom-supplied head dimensions but are still based on the ASME B18.5 specification.
|Bolt Diameter||Body Diameter||Head Diameter||Head Height||Square Width||Square Depth|
What is the shear strength of a carriage bolt?
The material used is much the same with carriage and stove bolts. However, carriage bolt sizes, stove bolt sizes differ and are the only specifications that make them different.
Carriage bolts vs. hex bolts are different as the heads are domed on the carriage bolts while the hex has an Allen wrench or hexagon fitting. Thread and dimensions can be the same for thread and depth, yet hex bolts won’t be countersunk or smooth when used on a table assembly.
Carriage bolts have a square neck under the head and a dome head with no drive. Bumber bolts are another name for carriage bolts. A307 low carbon steel is used in Grade 2 carriage bolts.
Carriage bolt’s strengths of 1/4 – 1 1/2 have a proof load of 33,000 psi and a tensile strength of 60,000 psi, while over 1 1/2 has a tensile strength of 55,000 psi but no proof load. Carriage bolts are completely on the shorter 6″, with 6″ of thread on lengths beyond 6″, and the rest being the non-threaded shoulder.
Carriage bolts have a square neck underneath the round head and a dome head without any drive. Tightening carriage bolts should be tightened from the nut side of the assembly. Carbon steel Grade 5 carriage bolts have a tensile strength of 120,000 psi and a proof load strength of 85,000 psi.
Carriage bolts are fully threaded up to 6″, with 6″ on carriage bolts lengths of thread over 6″, and the rest being the non-threaded shoulder.
Carriage bolts made of Grade 8 have a dome head, no drive, and a square neck under the head, making them the largest bolts.
The nuts tighten the carriage bolts. Hence this side must be accessible. The head of Grade 8 carriage bolts has six radial lines and is made of a medium carbon alloy steel that has been quenched and tempered. (Find the Best Flat Head Screwdriver Sizes)
They have a tensile strength of 150,000 pounds per square inch and shear strength of 90,000 pounds per square inch.
Stove bolt vs. carriage bolt is down to length. When you look at what are stove bolts, they are similar yet shorter.