Machine Bolts

What is the difference between machine bolts and machine screws?

Machine bolts are often confused with machine screws. Generally, machine screws are smaller in size compared to machine bolts, but are fully threaded. Machine bolts have a flat tip, not a pointed / tapered tip as seen in screws. Screws are typically used in pre-tapped holes or driven into materials for creating their own threads when tightened. In contrast, machine bolts are often paired with nuts at the end of the bolt for torquing during installation. Nuts are not generally mated with screws.

What are the thread fits or classes in these machine bolts?

Thread fit grades or classes of a fastener indicate the degree of looseness or tightness of mating threads. Generally, the higher the class number, the tighter the fit of the attached components (like nuts). Lower thread fit grades / classes are suitable for applications requiring quick assembly & disassembly. Higher thread classes are ideal for close tolerances, safety & strength.

What do the strength grades / classes mean in machine bolts?

• The grade or class of a machine bolt denotes its strength & compatibility with the respective nut.
• Generally, the higher the grade, the higher the strength of the fastener.
• SAE grades are used for Imperial-sized bolts. Bolts having the same SAE strength grade as a nut should be used together.
• ISO classes are used for Metric-sized bolts. Bolts having the same first digit as a nut are meant to be used together.
• ASTM grades are based on factors like strength, high pressures & high temperatures.