Q. How do you measure the length of a bolt or screw?

A. If the bolt or screw has a countersunk head (flat head) it is measured overall, from the top to the very bottom. All other bolts and screws (hex head, pan, round, etc.) are measured from under the head to the end of the fastener.

Q. What does the "grade" of a bolt mean?

A. The grade of a bolt indicates the tensile strength of the steel. The higher the number the harder it is. The most common grades are 2, 5 and 8. Grade 2 bolts are made out of mild steel and have no marks on the head, except maybe 'A 307A' .

Grade 5 bolts are a medium hard bolt and have 3 lines on the head. Grade 8 bolts are made out of alloy steel and heat treated to a real high tensile. Grade 8 bolts have 6 lines on the head. The specification number SAE J429 is the standard that covers these grades, and many other grades that are not as commercially available as these three are.

Q. How long is the thread length on a hex cap screw or hex machine bolt?

A. The formula that manufacturers use to put threads on bolts is 2 times the diameter of the bolt, plus 1/4 inch, up to 1 inch diameter or plus 1/2 inch over 1 inch diameter.

(2 x bolt dia.) + 1/4" = minimum thread length (bolt diameter less than, or equal to 1 inch)

(2 x bolt dia.) + 1/2" =minimum thread length (bolt diameter greater than 1")

An example would be a 1/2" diameter bolt would have 1 1/4" minimum thread (2 x 1/2") + 1/4" = 1 1/4" minimum thread length

Q. What does the grade of a metric bolt mean and how do they compare to SAE grades?

A. Metric bolts use the specification ISO 898 to specify the required mechanical properties for all the different classes of materials used to make threaded fasteners. In general there are 4 commonly used classes for externally threaded fasteners, 4.8, 8.8, 10.9 & 12.9 The larger the number, the harder the steel Class 4.8 is mild steel, used for machine screws, machine bolts, etc. Class 8.8 is a medium hard steel used for hex cap screws and hex socket screws. Class 10.9 is usually an alloy steel used for hex cap screws. Class 12.9 is used to make hex socket cap screws, set screws, etc. The following table shows the comparison between metric classes and SAE grades.