How to Recognize Metric and SAE Bolts

Posted on: by


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 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.
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.

Recognizing SAE and Metric car and truck bolts

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

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.

3 automotive bolts with classification markings

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

Bolt Markings
Tensile Strength
No markings on the bolt head indicate a low carbon steel bolt with a tensile strength 64,000 psi 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 105,000 psi. Four raised slots means a Grade Six, medium carbon steel, quenched/tempered, medium commercial quality, with 133,000 psi rating. Six raised slots is a Grade Eight, medium carbon alloy steel, quenched, tempered, 150,000 psi, 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 a bolt sizing tool from most any hardware store

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.

Line drawing comparing standard thread coarse and fine thread bolts

Standard coarse and fine thread bolts.

Metric and SAE
Fortunately, fastener manufacturers mark metric bolts differently than SAE bolts. Manufacturers emboss International Standards Organization (ISO) metric bolts larger than 6 mm 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.
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.

An alternate way of designating strength grade is the clock-face system.

Clock-face system

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 12 mm and that the thread pitch is 2 mm (there are 2 mm 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 (2 mm between threads is about 12.7 threads per inch).

Fastener manufacturers use bolt size designations to identify the size of a bolt or fastener

Bolt size designations.

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.
Steering clear of the pitfalls of automotive service saves time and aggravation. When you need procedures and specifications, a subscription to ChiltonDIY gives you full access to TSBs and Recalls, maintenance schedules, service and repair information.

29 responses to “How to Recognize Metric and SAE Bolts”

  1. fasteners manufacturers says:

    Great blog nice n useful information , it is very helpful for me , I realy appreciate thanks for sharing. I would like to read more information thanks.

  2. m88 says:

    Thank you for the sensible critique. Me and my neighbor were just preparing to do a little research on this. We got a grab a book from our local library but I think I learned more from this post. I am very glad to see such excellent info being shared freely out there.

  3. Peter says:

    Very informative. This is one of those things where a simple mistake could end up costing a lot more down the road. And if there is anything that you want to work, it’s the nuts and bolts that are holding your car together!

  4. Ryan Price says:

    Thank you very much for the kind words. I’m glad that my article was helpful and informative.

  5. cezarK says:

    I love to read your site

  6. Trobmacho says:

    Very good info that I wasn’t aware of. Never surprised at how much I don’t know.

  7. Carlos Garrison says:

    It is a very important topic to be careful about this topic. Thank you for aware us about these. Many auto workshops like Tire Mania Auto Repair also take their importance on this thing.

  8. Gombach says:

    Pretty! This was a really wonderful post. Many thanks for supplying this information.

  9. Dealer License Plates says:

    Take good care , a new follower .

  10. Ryan Lee Price says:

    Thank you :)

  11. "Kai Pettigrew" says:

    Thanks for share, you describe it very well

    • Ryan Lee Price says:

      Thank you Kai for the kind words. Nuts and bolts are an integral part of our cars that most times get overlooked. I’m glad you appreciated the article.

  12. Ryan says:

    Why don’t bolts have the exact sizes stamped on the head?(9/16 or 10mm) Do you know who regulates this?

  13. Ryan Lee Price says:

    It is more important to know the strength of the bolt and how much torque it needs than what size it is. Plus, most people that work with these kinds of bolts everyday know what size it is by merely looking at it.

    • Senahid says:

      Right you are. I am automotive machinist for about 10 years now and I can tell most smaller size bolts’ or screws’ size without measuring it. But I don’t know all the info that manufacturer does stamp on it.
      Thank you for very valuable info sir!

  14. roof repairs says:

    There’s certainly a great deal to learn about this issue. I like all the points you’ve made.|

  15. greg says:

    I have a hex bolt that only has a single large “S” stamped in the middle of the head. Do you know what that would indicate?


    • Chris Morris says:

      Hey Greg,

      It is usually a manufactures code. Nothing pertinent to the size or grade. As mentioned, markings on a bolt can tell you the tensile strength which is the minimum load at which the bolt can be expected to fail. To ascertain the load of your bolt, multiply this number by 100. That will give you the approximate tensile strength in Megapascals (MPa). From here you can also calculate yield load which is the tensile force that will produce a specified amount of permanent deformation (most commonly 0.2%) within a specific fastener. This is important when trying to determine which bolt has the strength and durability to withstand the type of fastening you need. Here is some more information on calculating this

Leave a Reply