All bolts fall into two categories: National Coarse (NC) and National Fine (NF). These two terms refer to the thread per inch count. For example, a 1/4-20 NC means the shank diameter is one quarter inch, with 20 threads per inch. A 1/4-28 NF means a quarter inch shank, with 28 threads per inch. A 1/4-20 iron bolt has a specified torque of 7 foot pounds. A 1/4-28 iron bolt has a torque of 9 foot pounds. Do not exceed these limits. Otherwise, internal cracking and eventual bolt failure will occur.
For a 5/16-18 grade 5 bolt, the recommended torque is 15 foot pounds. For a 5/16-24, the torque is 17 foot pounds. You must have an accurate torque wrench, and know how to use it, to prevent over-tightening the bolt.
For a 3/8-16 grade 5 bolt, the recommended torque is 30 foot pounds. For a 3/8-24, the torque is also 24 foot pounds. Some bolts, like the 3/8 shank, have the same recommended setting. However, they are not the same bolt. They should never be interchanged, since a stronger bolt can do the job of a weaker bolt, but not vice versa.
Bolt torque and breaking charts are available from the SAE. Thousands of bolts exist, and the SAE has painstakingly tested each size and grade. Design engineers rely on these charts for the size and strength of bolts. For example, a car's steering column is held on by bolts. If the wrong bolts are used, they could shear off, leaving the driver with no steering. Grade 5 bolts are a lot stronger than most people think. A 1/4-20 bolt is very small, but yet it has a breaking strength of 200 pounds. This is about the weight of a large person.