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The way to read the marking system is to add two to the number of marks. No marks on the head indicate a grade 1 or 2, three marks indicate a grade 5, four marks a grade 6, and six marks a grade 8. The first thing to know about grade markings is that no markings are considered to mean the fastener is made of mild steel. Conversely, the more marks on the head, the higher the quality and strength. Therefore, bolts of the same diameter will vary in strength depending upon the material they are made of and the number of threads per inch.
Hex Head Bolt Markings
The strength and type of steel used in a bolt should be indicated by a raised mark on the head of the bolt. The type of mark depends on the standard to which the bolt was manufactured. Most often, bolts used in machinery are made to SAE standard J429, and bolts used in structures are made to various ASTM standards. The accompanying tables give the head markings and some of the most commonly needed information concerning the bolts. For further information, see the appropriate standard.
There are often "extra" marks on a bolt head-marks in addition to those shown in the charts-and these marks usually indicate the bolt's manufacturer.
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Please note that ASTM A325 Type 2 bolts have been discontinued, but are included in the chart because they can be found in existing structures. Their properties can be important in failure investigations. While this isn't germane to automotive purposes, I decided to err on the side of completeness.
While the bolts shown are among the most common in the U.S., this list is far from complete. In addition to the other bolts covered by the SAE and ASTM standards, there are a host of international standards, of which ISO is perhaps the most well known.
A more detailed explanation of bolt torque specifications and additional related information on these and other automotive fasteners is covered in depth in my book, Garage and Workshop Gear Guide, published by Motorbooks and available from your favorite bookseller.
So, hopefully, you now know a whole lot more about threaded fasteners and what those markings on their heads mean.
Log onto boltdepot.com and click on the Printable Fastener Tools link for an assortment of useful charts, tools, measuring devices, thread sizes, and other handy items you can print out for free. These are great to have on hand for reference in your garage.