Metric bolts have become a hot topic among vehicle repair technicians and manufacturers since since Ford’s Pinto engine and Cadillac’s 8.2 L Eldorado engine were introduced in the 1970s. Metric bolts are now preferred by the majority of domestic autos over SAE inch-sized bolts. Metric bolts must be used in your new car rather than the usual SAE bolts. The purpose of this post is to give you a simple manual for identifying your bolts.
It should go without saying that bolts come in a variety of sizes, shapes, materials, tensile strengths, diameters, wrench sizes, and pitches. It is already obvious that no two bolts are exactly alike. An automotive repair maker can determine which bolt is specialized for which use thanks to these characteristics. A long bolt could get in the way of your car’s other components. On the other side, if a small bolt is used somewhere else when a long bolt is needed, there is a danger that it will break. The incorrect bolt placement will not only impede your car’s motor from operating to its full potential but will also result in a variety of unwelcome problems for both your automobile and your wallet.
The type of bolt is identified by a marking on the bolt. The tensile strength and the amount of force that a specific bolt can withstand are stamped on the bolt’s face. Although a bolt with a high tensile strength should be able to handle more torque, it is still advisable to keep a torque chart close by and adhere to its recommendations.
You may have no idea that your bolts might be either hard or soft. So first, you should be aware of this if you ever see a bolt without any markings. A low carbon steel component with a tensile strength of more or less 64,000 psi won’t be indicated by a bolt. They are referred to as “soft bolts” if you discover one. Whenever there is a need for light manufacturing, soft bolts are typically used.
Grab a bolt and examine the raised slots on its body first. If there are three slots, these are Grade Five slots with a 105,000 psi tensile strength. View the graph below:
If you ever find yourself unsure of which bolt is your “eight grade” and which is a “grade six,” you can always pick up a size tool from your neighborhood hardware store.
Your number of threads per inch will increase as the pitch decreases. American bolts come in two different varieties: United National Fine Thread and United National Coarse Thread. The distance between the crest of one thread and the same location on the crest of the following thread is known as the thread pitch. You will be able to identify the type of your bolt if you comprehend this.
A coarse thread is less likely to strip since it turns in and out more quickly. However, fine threads enhance torque, which improves their ability to hold.
- Different markings are used on metric bolts. Bolts marked “M” or “ISO M” on top have a diameter more than 6 mm. Additionally, metric bolts may be identified by numbers like 5.8 or 10.9. They are the bolt’s size but not quite as strong. The strength of the bolt will increase with the higher the number.
- Another method to tell a Metric bolt from an SAE bolt is by size. If your metric bolt is size M12X2, the threads on it have a main diameter of 12 mm, and their pitch, or the space between them, is 2 mm. Based on the separation between the threads, ISO categorizes metric threads.
Sorting bolts by size, shape, and measurement makes it simple to distinguish between various types of fasteners. It can be nerve-wracking to figure out how to tell a Metric bolt from an SAE bolt made in the US. However, if you know which bolt to use to repair your car, you can make it run like Usain Bolt did again.
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In This Article...
How are metric and SAE bolts different from one another?
People have learned that measuring things is essential to their survival since the beginning of time. There are now two recognized systems in the United States: standard and metric. Although it can be a little perplexing, we are still hesitant to fully adopt metric alongside the rest of the world. So what is the difference between metric and standard? Well, it was a bit lengthy.
A Story in Two Measures
American customary units were initially adopted from the British Imperial System. The Society of Automotive Engineers created the SAE system, which is still widely used in the automotive industry. However, France started working on a system that would measure in perfect multiples of ten in the late 1790s, which would make calculations and conversions considerably easier. This was the Metric System, which eventually gained widespread acceptance. The U.S. has resisted for economic reasons, but this is unusual, especially in a global economy when one system is used as a benchmark globally.
Better Off Thread
Fasteners are measured and referred to differently in metric and SAE standards. Thread count, often known as TPI (threads per inch), and length are the parameters used to measure SAE nuts, bolts, and screws. The length and pitch, or the separation between the threads, of metric fasteners are measured. Therefore, in a way, the systems are opposed: Because there is more crammed into a given space, conventional bolts with higher TPIs have finer threads. On metric fasteners, the pitch determines how far apart the threads are from one another. The metric system offers up to four different pitch options, depending on the situation. Metric threads are convenient since they convert to distance with ease. One full turn of a fastener changes the exact distance of the pitch by millimeters, making it more simpler for engineers to handle during the design process.
Pitch Perfect
Experienced technicians who frequently work with both types of fasteners might be able to distinguish between metric and SAE bolts by looking at them, although the distinction is subtle because most threads are quite small. And making a mistake has consequences. According to the application, forcing a metric bolt into an SAE nut may strip one or both, impair the connection’s integrity, and may even result in leaks. It will also make some repairs far more difficult. Similar to using an SAE wrench to tighten down a metric bolt, doing so can soon round out the head and cause a major issue.
Always make sure you are using the appropriate tools for the project and never force a fastener into place as you run the danger of cross-threading. Although some fasteners have stamps with specifications, it is advisable to use brand-new hardware from a container with markings. You can very much count on looking at metric if you’re working on a Japanese, Korean, or German vehicle. Most American manufacturers previously utilized SAE, but modern cars with parts from other nations are starting to use metric as well. As a result, always check your owner’s handbook.
To identify threads at home, thread gauges are readily accessible and can be printed or cast in plastic. It is crucial to understand what you’re working with when seemingly insignificant discrepancies can suddenly cause a world of trouble.
For routine maintenance and repairs, visit one of our 17,000 NAPA AutoCare facilities or have a look at all the tools & equipment on NAPA Online. Visit your neighborhood NAPA AUTO PARTS store to speak with a qualified professional for more details on metric vs. standard.
American automobiles: metric or standard?
While foreign cars almost entirely utilize metric dimensions, American cars are constructed with nuts and bolts that use conventional measurements.