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 Bolt Designations
ASTM
standard |
Size
range |
Tensile
strength,
ksi |
Material |
Head marking |
| A307 |
1/4 thru 4 |
60 |
Low carbon steel |
 |
A325
Type 1 |
1/2 thru 1
1-1/8 thru 1-1/2 |
120
105 |
Medium carbon steel,
quenched & tempered |
 |
A325
Type 2 |
1/2 thru 1
1-1/8 thru 1-1/2 |
120
105 |
Low carbon
martensite steel,
quenched & tempered |
 |
A325
Type 3 |
1/2 thru 1
1-1/8 thru 1-1/2 |
120
105 |
Weathering steel,
quenched & tempered |
 |
| A449 |
1/4 thru 1
1-1/8 thru 1-1/2
1-3/4 thru 3 |
120
105
90 |
Medium carbon steel,
quenched & tempered |
 |
A490
Type 1 |
1/4 thru 1-1/2 |
150 |
Alloy steel,
quenched & tempered |
 |
A490
Type 3 |
1/4 thru 1-1/2 |
150 |
Weathering steel,
quenched & tempered |
 |
Often one will find "extra" marks on a bolt head--marks
in addition to those shown above. Usually these marks indicate the bolt's
manufacturer.
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.
Bolt information links
