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Many beginner mechanics don't know the difference between SAE and Metric bolts, and it's very important that they do to avoid cross threading and damaging threads, as well as rounding the fastener head. Many SAE and Metric bolts have similar threads, specifically an SAE 3/8"-16 thread is very close to a Metric M10x1.5 thread, which will start to thread by hand, but once fully threaded and torqued, the threads will damage. Soft materials, such as aluminum heads or blocks can get damaged very easily with an incorrect threaded bolt installed. Knowing the grade of the bolt will allow you to properly torque the bolt. A typical hex head bolt, as shown in the image above, will have no markings, hash marks, or numbers. Typically, a bolt with no markings or hash marks will be an SAE bolt, while bolt heads with decimal numbers will be Metric. This doesn't always bold true, but is correct most of the time. Both the hash marks and decimal numbers denotes thier grade, or strength. More hash marks means a stronger bolt, while a higher decimal number means a stronger bolt. Aftermarket performance bolts, such as ARP ® will not have the same universal markings, but may have thier own brand grade, such has 8740, ARP2000 ®, etc. Now that you know the grade of the bolt, you can properly set your torque wrench to achive proper bolt torque, which is is the clamping force. Bolts are designed to slightly stretch, and thats when they are the strongest. A bolt that isn't stretched enough (torqued too little) will loosen, while one that is overstretched (torqued too much) will fail. By torqueing all your bolts, an evenly distributed load will be made, keeping everything parallel and leak free.
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