ASTM A325 bolts are very similar to SAE Grade 5 bolts in regards to their chemical and mechanical properties. The main difference between the two can be found primarily in the applications that they are specified for as well as their configuration. Since Grade 5 is a specification falling under the Society of Automotive Engineers, they are normally used in common OEM or automotive type applications. A325 is an ASTM specification and is used for structural steel connections.
A325 structural bolts are typically heavy hex bolts, whereas Grade 5 bolts are most typically made, and are readily available, in a standard hex bolt configuration. With the F3125 specification now governing the A325 specification, alternate head styles are allowed, provided the heads are stamped “A325S”. Another difference is the diameter range that each grade covers. Grade 5 bolts are available from 1/4″ diameter up to and including 1-1/2″ diameter. The A325 specification covers bolts from 1/2″ up to and including 1-1/2″ diameter maximum.
The standard thread length which both grades come with also differ. Grade 5 bolts, similar to most grades, follow the standard thread length formula of two times diameter plus 1/4″ or 1/2″, depending on if the bolt is up to or over 6″ in length. A325 bolts have specific shorter thread lengths based on the diameter of the bolt, regardless of the overall length of the bolt.