FTM 157: Thread Stick-Out

This edition of the Fastener Training Minute with Carmen Vertullo was originally published October 18, 2020 as “Thread Stick-Out” during episode 157 of Fully Threaded Radio.

Todays Fastener Training Minute comes by way of an email from one of our clients, and that email had to do with a question about “thread stick-out“. Now “Stick-Out” doesn’t sound like a very technical term, but it actually is the technical term used to describe the number of threads that should protrude beyond the nut surface after we install the nut on the bolt. Sometimes this is called minimum thread protrusion, but its much easier to say Thread Stick-Out, so I am going to stick with stick out.

So, stick-out is the amount of threads or number of threads that should protrude beyond the nut once the nut has been installed and tightened down correctly. There are a couple of different scenarios under which we need to be concerned with this. The first one obviously is just because the standard hex nut is not very thick. We don’t really get very many threads engaged maybe less than a diameter of threads in a standard hex nut. And so we need to make sure that all those threads are available to do their work. And when you consider the fact that most bolts have a chamfer on the end of the threads, that means that the chamfer could give us one to two threads that are not fully formed.

Also, the nut has a chamfer which could give us about a half a thread that’s not fully formed. So we have some number of threads that aren’t available to do their full work. So from an engineering perspective, most assembly instructions want to see 2 to 3 threads protrude beyond the nut. That’s pretty much the standard 2 to 3 threads. More than 3 and you are wasting threads and it looks kind of ugly. Now interestingly enough, we also have this issue when we use nylon insert lock nuts. Beyond the metal part of the nut we also have some nylon which must be engaged and unless we have both threads in that nylon, it cannot do its work either. So we want to make sure that there are fully formed threads in the nylon. Once again about two threads beyond the end of the nylon insert lock nut would be the minimum to absolutely ensure that

And again more than that, it looks kind of ugly. A lot of folks like to have the nut just right up to the edge of the surface of the nylon because that looks kind of nice but you do risk not having fully engaged nylon. The only place that I know of where we see a requirement for thread stick out is in structural bolting, That comes from the research Council on structural connections and they always use heavy hex nuts. Their requirement is that no threads have to stick out as long as the end of the nut is flush with the end of the bolt. Some threads can stick out but you don’t have to have any stick out. Also, I know from my background in the military and aviation that they want to see two or three threads sticking out for the various reasons that I said before. There’s some chamfer on the bolt and there is some chamfer in the nut and we want the threads in the nut to be fully engaged.

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