Story and Photography by Ryan Lee Price

Since Ford’s introduction of the Pinto engine in the ’70s and Cadillac’s badging of the 1970 Eldorado as an 8.2L engine, the use of the metric system has become more prevalent in domestic vehicles, and odds are good that the majority of those domestic vehicles on the road today have more metric fasteners than the inch-size (SAE) type. While the size may be similar, the pitch of the threads will be different, so it is important to be able to recognize the difference between metric and standard fasteners.

There is nothing in greater quantity on your car than the nuts, bolts and screws that help hold it together.

Fastener manufacturers identify bolts and capscrews by type, length, major diameter, pitch (threads per inch), length of thread, class or fit, material, tensile strength, and the wrench size needed to tighten them. All these variables match the fastener to the particular application for which it was designed. A bolt which is too long might interfere with other parts and won’t utilize the full design of the bolt. A capscrew which is too soft might snap off before the proper torque is reached. The wrong thread size will strip out the nut or receiving hole and cause a host of new problems.

Bolts and capscrews are not all made of the same quality material nor is the tempering the same. Markings are stamped on the face to indicate the bolt type, and specifically, show the tensile strength of the fastener, or the amount of pull an object will withstand before breaking. Generally, a fastener with more tensile strength accepts more torque before breaking, but to avoid breaking bolts, a torque chart is most helpful.

Bolt manufacturers use a variety of methods for marking the classifications of their products.

Tensile Strength

No markings on the bolt head indicate a low carbon steel bolt with a tensile strength 64,000psi or less. These are "soft bolts," commonly referred to in mechanical engineering circles as a Grade Two or Three bolt. Soft bolts have an indeterminate quality and are commonly used in light manufacturing. A bolt head with three raised slots “stamped” on it indicates a Grade Five, medium carbon steel, tempered; it is a minimum commercial quality fastener with a tensile strength of 105,000psi. Four raised slots means a Grade Six, medium carbon steel, quenched/tempered, medium commercial quality, with 133,000psi rating. Six raised slots is a Grade Eight, medium carbon alloy steel, quenched, tempered, 150,000psi, and is the best commercial quality. A Grade 12 has eight raised slots surrounding an oval, and is a special alloy steel, quenched and tempered. A Grade 12 is recommended for critical use and competition purposes.

If you’re stuck with a pile of unsorted bolts, instead of pulling out every wrench in your toolbox, get one of these simple sizing tools from most any hardware store.

Coarse and Fine Threads

For American bolts (standard inches) there are two basic types: The Unified National Fine thread and Unified National Coarse thread variety. Thread pitch, or the distance between the crest of a thread to the same spot on the crest of the next thread, helps determine the type; the smaller the pitch, the greater number of threads per inch. Use a thread-pitch gauge to determine the number of threads-per-inch. Use coarse-threaded hardware in cast iron and aluminum because it won’t strip the mating hole as easily as fine threads. Coarse-threaded hardware screws in and out more quickly and is less subject to stripping and galling (the threads ripping particles of metal from each other thereby damaging both threads). Fine-threaded hardware tends to take more torque and, as a result, has a slightly better holding capability.

Metric and SAE

Fortunately, fastener manufacturers mark metric bolts differently than SAE bolts. Manufacturers emboss International Standards Organization (ISO) metric bolts larger than 6mm in diameter with either “ISO M” or “M” on top of the head. In addition, manufacturers stamp most metric bolts with a number on the bolt head, such as 4.6, 5.8 or 10.9. This number has nothing to do with the size, but does indicate the relative strength of the bolt: the higher the number, the stronger the bolt. Manufacturers mark some metric nuts with a single-digit number to indicate the strength, and some emboss the "M" and strength grade on the flats of the hex.

The Clock-Face System

An alternate way of designating strength grade is the clock-face system. Fastener manufacturers mark the external faces of the bolt and/or nut with a dash at the appropriate hour mark corresponding to the relative strength grade. One dot at the 12 o'clock position indicates a grade under 12, while two dots found at the 12 o'clock position is Grade 12 and over. Manufacturers indicate specific grades with tick marks: a tick mark at six o’clock is a Grade Six bolt, eight o’clock is Grade Eight and a tick mark at two o’clock (with two dots at noon) means a Grade 14.

Size Designations

Fastener manufacturers identify the size of a metric fastener differently than an SAE fastener. As an example, a metric fastener size is: M12 x 2. This means that the major diameter of the threads is 12mm and that the thread pitch is 2mm (there are 2mm between threads). ISO classes metric threads by the distance between the threads, and the distance between threads does not exactly correspond to number of threads per inch (2mm between threads is about 12.7 threads per inch).

Save Time and Aggravation

Time spent organizing bins of nuts and bolts into types and sizes will save a lot of time on the other end, not to mention the headache caused from mistaking a metric bolt for a U.S.-spec bolt.

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Not only is Ryan Lee Price a freelance writer specializing in automotive journalism and a former long-time magazine editor, he is part of the technical editorial team that provides content for most all of the Chilton products. He currently resides in Corona, California, with his wife Kara and their two children.