Diameter is the most predominant way to initially identify a bolt, followed by secondary information such as the length, thread pitch, tensile strength, and drive type.
However, the diameter of a bolt is often confused with the size of the wrench that is used to drive the bolt. For example, the valve cover bolt for most domestic cars is a 1/4″ bolt that has a 3/8″ hex head. A rookie will often identify this bolt as a 3/8″ since that is the wrench that fits it. In reality, the rookie just asked for a bolt that is about three times larger than what he wants.
Standard bolt diameters fall on specific fractions of an inch. The most common bolt diameters used in the automotive industry are 1/4″, 5/16″, 3/8″, 7/16″, 1/2″, 9/16″, and 5/8″.
Note that standard fasteners below 1/4″ are called machine screws. Machine screws are covered in a separate lesson – Machine Screw Identification.
Measuring a bolt’s diameter with a digital caliper (as shown in the image to the left) is quick and easy, but there are a few things to keep in mind.
Always measure across the threads. Some threads are formed by a rolling process and the shank of the bolt will be slightly smaller than the threads. So, always measure across the threads for accuracy.
The measured diameter will be less than the “sold as” diameter. The actual diameter of a fastener will always measure slightly smaller than the bolt’s “sold as” size. Clearance is built into the fastener’s sizing so a fastener can go through a hole that is the same size. For example, a 3/8″ fastener will go through a 3/8″ hole since the fastener is made slightly smaller.
Don’t confuse standard bolts with metric bolts based solely on the measured diameter. For example, a 5/16″ bolt has a “sold as” size of .312″ and an 8mm bolt’s “sold as” size is .315″. In this example, there is only .003″ difference between the two bolts. it would take additional information, such as the thread pitch or head markings (grade/class) to positively identify the bolt.
The length of a bolt is measured from the end of the bolt to the bottom of the head. It does not include the thickness of the head. The length of a standard bolt is typically offered on major fractional increments with the increments getting further apart at longer lengths. Just like with the “sold as” diameter, the measured length of a fastener will be slightly shorter than the “sold as” length.
Thread pitch is the number of threads per inch (TPI) as illustrated in the image to the left. Standard fasteners are available in two pitches, coarse and fine. The thread pitch will vary among the bolt sizes; however, the coarse and fine pitch for each size bolt is standardized as shown in the Standard Bolt Specifications Chart at the bottom of this lesson.
The coarse pitch (considered to be the standard pitch) is the most common pitch and performs well overall. Some of the characteristics of the coarse pitch are:
- The threads of a course pitch bolt are deeper than a fine pitch bolt so it holds better in soft metals, such as aluminum.
- The threads of a course pitch bolt have a steeper angle than the threads of a fine pitch bolt. This results in slightly less clamping power for the same amount of torque as a fine pitch bolt,
The fine pitch is typically used in special applications where maximum clamping power and precision is needed. Some of the characteristics of the fine pitch are:
- The threads of a fine pitch bolt are not as deep as a course pitch bolt so it tends to strip out of soft metals, such as aluminum.
- The threads of a fine pitch bolt have a flatter angle than the threads of a coarse pitch bolt. This results in a higher clamping power for the same amount of torque as a course pitch bolt,
Measuring Thread Pitch
Thread pitch is measured with a thread pitch gauge as shown below. The gauges are placed against the threads to find the one that matches. Note that the gauge shown in the illustration is actually a metric thread pitch gauge.
YouTube – Using a Screw Pitch Gage
Standard bolts are also graded on the amount of tensile force they can withstand before breaking (effectively – pulling apart). The Society of Automotive Engineers (SAE) developed a grading system for the tensile strength of standard bolts used in the automotive industry (Grade 2, Grade 5, and Grade 8). Their head identification markings are shown in the image to the left (Add 2 to the number of radiating marks to determine the grade).
- Grade 2 – No Head Markings – Low or medium carbon steel – Min. Tensile Strength 74,000 PSI
- Grade 5 – 3 Lines on Head – Medium carbon steel, quenched and tempered – Min Tensile Strength 120,000 PSI
- Grade 8 – 6 Lines on Head – Medium carbon alloy steel, quenched and tempered – Min. Tensile Strength – 150,000 PSI
Hex Head (Cap Screw)
The hex head (cap screw) is the default drive for bolts. If you order a bolt and do not specify a drive, you will get a hex head drive. The size of the hex is often confused with the overall size of a fastener. However, a fastener is identified by the diameter of the threads, not the size of the hex head. For example, a 3/8″ bolt has a 9/16″ hex head.
The common hex head sizes in the automotive service industry are: 5/16″, 3/8″, 7/16″, 1/2″, 9/16″, 5/8″, 11/16″, 3/4″, and 13/16″
The Phillips drive is more associated with machine screws (standard fasteners smaller than 1/4″) than with bolts; however, this drive is sometimes used on small bolts such as 1/4″.
The Phillips drive is available in many sizes from #0000 to #4, With #2 being the most popular size.
Hex Key (Allen)
The hex key drive is used sporadically in the automotive industry. They are often used in close quarters locations since they can be driven from the center. Like all internal drive heads, there is also less chance of a tool slipping and causing damage to the dash or interior.
Some of the smaller hex key sizes are used for trim and dash applications. Some of the larger sizes are used for brake hardware and on drain/fill plugs.
Torx – Standard
The Torx (the original brand name) drive, is often called a star bit. It was designed to allow more torque to be transferred to the fastener without slipping. The drive was developed in 1967 but it didn’t show up in the automotive industry until the 1990s.
Torx bolts were initially used on certain systems (such as seat belts and airbags) to keep the average consumer from working on them. Torx bolts are now being used in more areas of a car, including the suspension and brake systems.
The Torx drive platform includes internal Torx bits and external Torx sockets. The most popular of the Torx bits are T6, T10, T15, T20, T25, T27, T30, T40, T45. T50, and T55
Torx – Safety
The Torx safety fastener has a pin sticking up in the middle of the hole which blocks the use of a standard Torx bit. The Torx safety drive requires a special Safety Torx bit which has a hole in the center.
Safety Torx bolts are often used on seat belts and supplemental restraint systems (SRS) to help prevent the public from tampering with the systems.
The triple-square bit, also known as XZN, is a 12-point spline. Triple squares are commonly found on German vehicles such as BMW, Mercedes, Porsche and those from the Volkswagen Group. These fasteners are often used in high-torque applications, such as cylinder head bolts and drive train components.
Sizes are M4, M5, M6, M8, M9, M10, M12, M14, M16, and M18. Note the “M” does not stand for metric. Triple square is not a standard or metric tool.
Standard Bolt Specifications
|1/4″||NC||20||.250″||7 Lb-Ft||11 Lb-Ft|
|1/4″||NF||28||.250″||9 Lb-Ft||12 Lb-Ft|
|5/16″||NC||18||.312″||15 Lb-Ft||21 Lb-Ft|
|5/16″||NF||24||.312″||16 Lb-Ft||23 Lb-Ft|
|3/8″||NC||16||.375″||26 Lb-Ft||40 Lb-Ft|
|3/8″||NF||24||.375″||30 Lb-Ft||42 Lb-Ft|
|7/16″||NC||14||.437″||42 Lb-Ft||60 Lb-Ft|
|7/16″||NF||20||.437″||47 Lb-Ft||70 Lb-Ft|
|1/2″||NC||13||.500″||65 Lb-Ft||95 Lb-Ft|
|1/2″||NF||20||.500″||75 Lb-Ft||110 Lb-Ft|
- NC – The Unified National Coarse (UNC) is a defined standard set of thread forms, sizes, and other variables commonly used in the United States and Canada.
- NF – The Unified National Fine (UNF) is a defined standard set of thread forms, sizes, and other variables commonly used in the United States and Canada.
- TPI – Threads Per Inch
- Decimal Size – The decimal equivalent to the fractional size of a bolt. Note that the actual bolt diameter will be slightly smaller than the calculated decimal equivalent.
- Torque Specifications – These specs are for a “standard” bolt. Bolts designed for special applications may vary. If any doubt, check the manufacturer’s specifications.