How many threads should show?

[Art-n-Marge]

This is a question about nuts and bolts. I remember reading this or hearing this somewhere but I can't find it any more.

How many minimum threads or distance are supposed to be exposed when driving a bolt through a nut? For example, if you install a nut on a bolt and there are NO threads showing, this is a no-no and I undertand that this condition can cause a failure especially under stress conditions because the nut does not have enough bite on.

I have a couple of applications where the threads are showing way too far and some way too short. I have the option of cutting the long bolt to make it shorter but still protrude past the nut surface. And for the bolts too short I will get longer ones so they protrude enough.

I just don't know how much.

On edit : changed to "install a nut on a bolt"

[Tom S.]

As long as the thread is through the nut, you should be fine. Extra thread does nothing to add strength. I do remember form my long distant apprenticeship days that minimum standard was 1/2 the diameter of the thread, ie., 1/4" on a 1/2 bolt. Make sure when you cut it to coat the end with something to prevent rust (grease works well if the bolt is somewhere you won't rub against it). I don't know how you plan on cutting it, but if it's a graded bolt, be careful not to get it too hot.

[bigmurf]

It is often said that two threads must be exposed above a nut. The reason for this is that the first two threads of a bolt are often poorly formed, and may not engage the nut properly. If they're not doing their share, the other threads in the nut will be overloaded, and the nut may strip.

[Art-n-Marge]

Tom for the cutting, I decided not to worry about it, but I have been known to use a 4.5 inch diameter grinder "cutting" disc (very thin). I do not cut grade 8 bolts, I prefer to change the size or leave them alone.

If I use my small grinder, would that get too hot? I don't know how to tell.

Thanks Bigmurf. I recall something like two threads, but this does not sound long enough in my application. I thought there was a ratio between the size of the nut and the protrusion. For example, it the nut is 1/4" tall then the protrusion is the same or half the length or some number, because of differing thread sizes - coarse versus fine, etc.

[Tom S.]

Art, A grinder with a cut off wheel will get the bolt hot enough to change the temper. If that's an issue, cut a bit, poor on some water, and repeat until you are done. DO NOT POOR THE WATER ON THE CUTTING WHEEL! Visually, once a metal has turned color, you are affecting it's heat treating.

[rames14]

Ah, the good old Apprenticeship days. Big Murf, I think you are meaning that you should have two threads exposed because if they were in the nut (and not sticking out), they wouldn't equally hold the torque load. This depends partly on how the threads are cut or formed (rolled, cut, etc). For higher quality bolts, the tolerance is such that this should not be a problem, but still a good rule of thumb. Definitely if you see a color change - starting at a very light straw color, the microstructure has changed and no longer has the same mechanical properties. However, mechanical properties can start changing as soon as 400 degrees. Not cutting on grade 8 bolts is a good idea.

[grampachet]

Drive a nut through a bolt :>)?

[8e3k0]

A "Zip" disc can be used on grade 8 bolts with no harm and very little heat and they cut very fast. These discs are ultra thin for cutting only. You must be careful and not twist or change the angle when cutting with these discs as they can be ruined very easily.