Hex Head Bolt Markings
The strength and type of steel used in a bolt is supposed to 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 tables below give the head markings and some of the
most commonly-needed information concerning the bolts. For further
information, see the appropriate standard.
ASTM A325 Type 2 bolts have been discontinued, but are included above because they can be found in existing structures. Their properties can be important in failure investigations.
While the bolts shown above are among the most common in the U.S., the list is far from exhaustive. 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.