Nuts and bolts (sometimes called studs) are some of the most secure ways to attach two things together, and if you want to make sure they last a long time, a good thread locker can do wonders. While screws can accomplish the task, with much less preparation and effort, nuts and bolts are typically longer lasting and more secure. They’re also more commonly used when the two things you’re attaching will need to be taken apart. It can sometimes be difficult to determine when you have properly secured a bolt. That’s where the three-thread rule comes from.

The three-thread rule is generally considered to be the minimum amount of thread engagement you need for a secure hold with a bolt. Under the rule, you should thread the nut and rotate it around until you can see three rotations of the bolt’s thread protruding out the other side. This ensures that you have enough engagement for the threads to hold securely. 

The reason for this is that often the threads at the very end of the nut or bolt are less structurally sound than the threads further in. They can start to strip or break down the closer to the edge you get. By attaching the nut so that three full revolutions of threads are sticking out, you’re ensuring that the maximum amount of surface area on the bolt is in contact with the maximum amount of surface area of the nut. If you need to get it off, penetrating oil can help with that.

Another rule for nuts and bolts is determined by the material you’re working with. If you’re attaching steel nuts and bolts together, another rule suggests that you should have as much of the bolt protruding from opposite end as one diameter of the bolt. For example, if the bolt your attaching is a three-quarter-inch bolt, the threads should protrude out by about three-quarters of an inch. The rule differs for aluminum fasteners; it says you should go for two times the diameter of the bolt.

All of these rules are designed to help you find the sweet spot for fasteners. You don’t want to have too little thread engagement, or the nut may not hold at all. You also don’t want to have too much, lest the end of the bolt sticking out become problematic. Having a solid rule like this helps you find the happy medium of thread snugness, and bolt protrusion.