Bolt Identification Bolthead Markings Bolt Torque Specs

Most bolts are stamped with either a number (metric bolts) or a series of lines (American bolts also known as fractional bolts).  These numbers or markings indicate their minimum strength or, technically speaking, their property class.   In addition to these markings, most reputable bolt manufacturers will also stamp their trademark in the head of the bolt, which allows for easy identification and accountability.  In most cases, bolts used in machinery are made to SAE standard J429, and bolts used in structural applications are made to ASTM standards.  However not all bolts are stamped.  As an example, the hardware manufactured for OEM purposes do not require markings.  The importance of what is stamped on the head of a bolt relates to the job the bolt will do.  First if a bolt is a replacement of an existing bolt you wouldn't want to substitute the bolt for a weaker bolt compromising strength.  Another reason is if you know the property class of an existing bolt and you know its diameter, you can determine the suggested basic torque setting, even if you don't have a reference to the original torque recommendations.  Typically replacing a lower grade bolt with a stronger bolt is generally acceptable however there are instances when a lower grade bolt is used to handle certain types of loads that might shear a harder bolt.  If quality and strength is a concern, stick with marked bolts.