Survivalism & Self Sufficiency Topics > Do It Yourself - Projects, Ideas and How To

Beginners guide to nuts, bolts and threading

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ohio oz:
Hi everyone, new listener to TSP, I took it to heart when Jack said to start talking about the things you are passionate about.  I am a machinist and maybe even an all around handyman, and I'm finding myself frustrated that many people I meet can't figure out a lot of seemingly basic things (nobody on here, of course). Here's my contribution to some of the DIY projects I see some of you doing.  While you can't eat nuts, bolts or drills, you might save a few trips to the hardware store, or the repairman, and that money saved can feed the family.

If you like this, let me know, I have some other subjects in mind.

Identifying nuts and bolts for those odd projects can be confusing for the beginner, so I wanted to do a brief tutorial on nuts and bolts basics.  I also wanted to show how a simple threaded hole is made and a repair that might give new life to something that is broken or worn out.  For many of you, these will be things you already know, but I’ve found it difficult to get good basic information on things like this without lots of jargon and digression  into specialized cases.
The following images will look at a grade 5 3 inch 3/8-16NC hex bolt.  What does all that mean?
The length of a bolt is measured from under the head to the end of the threaded shank; in this case, we have a 3 inch bolt.

The diameter of the bolt is measured across the largest part of the threads.  Bolts are generally made slightly smaller than their nominal diameter for ease of installation.  Here we have a 3/8 inch bolt, and as you can see, it measures .370”.  In theory this bolt would measure .375” there is a difference of .005, about the thickness of a hair.

The hex head of this bolt measures 9/16”.  This is the size wrench or socket you would need to turn this bolt. There are a huge variety of heads (square, socket, 12-point), so it is necessary to specify that this bolt has a hex head.

This is a bit hard to show due to the small size and some camera parallax, but this bolt has 16 threads per inch.  The NC in 3/8-16NC stands for National Course, which refers to the angle and shape of the threads.  We could also go to the hardware store and pick up a 3/8-24NF bolt, where the NF means National Fine.  There are other standardized thread angles and shapes for special cases but for 99% of common uses, you will see NC and NF threads. 

Some further inspection of the bolt head shows us three radial marks, and the manufacturers stamp.  Strangely, if a bolt or nut shows no markings it tells us the bolt is grade 2, three marks means the bolt is a grade 5, and five marks indicate that the bolt is grade 8.  The grade is basically a measure of the strength of the steel used to make the bolt. There are specific industry standards for each grade, and even standards above and beyond grade 8.  The fasteners you will pick up at the local hardware store will probably   be grades 2, 5 or 8.  The three marks indicate that that this particular example is a grade 5 bolt.

Now that we have identified our bolt as a grade 5 3 inch 3/8-16NC bolt, what can we do with it?  Well if we were boring, we could find the corresponding nut and screw them together, but we need to fix or create something. 
For this I will need:
A hammer
A center punch
A center drill
A 5/16” drill bit
A 3/8-16 tap
A tap handle
I also used my cordless drill to drill the holes.  If you happen to have a drill press, even better!

As an example, I’m going to make a hole to fit this particular bolt.  Since you can’t just buy the widget I happen to need with the correctly threaded hole in the correct place at the hardware store, I’m going to drill and tap it to fit my needs.  The first step is to center punch the location of the hole.

A sharp whack with a hammer and I have marked the center of my hole.  Why not just start drilling you ask?  Because I need this hole to be exactly in this location, if I try to drill the hole right away the drill can move around as it spins.  Once the drill gets started it is nearly impossible to correct its course.

The next step is to drill a small center hole.  Unless you are making a very small hole, your drill can still move away from the center punch mark.  In comes the center drill.  The center drill is very short, with a small drill point on the end, followed by a taper to guide the correct sized drill into place.  You can see the center drill to the left of the center hole.

Now that we have a center hole in location, we drill with the correct sized bit.  How do we determine the correct size?  We could start with the bolt diameter, the number of threads per inch and the angle of the threads and do some math, but the easiest way is to refer to a handy little chart put together by any of several tool companies. According to the chart we need a 5/16” drill.  I’ve highlighted the pertinent information.

The 5/16” drill fits snugly through the corresponding nut, so it stands to reason that this is the correct size.

Our drill makes quick work of the hole.  I should note, it is important to get each bit perpendicular to the surface you are drilling.

This is a 3/8-16 tap; imagine it as a really hard bolt with some areas cut away for the chips made by cutting the threads to fall through.  If you look closely, you will see that there is a taper on the end to gradually cut each thread.

A tap has a square shank and is loaded into a tap handle.

We tap (cut internal threads in) the hole by applying downward pressure while twisting the tap handle in a clockwise direction.  Tap handles are shaped the way they are so you can apply force on both sides at the same time.  If you only pull on one side of the tap handle it will try to bend and possibly break the tap.  Because taps are so hard, they are brittle and will break easily.  A broken tap is one of the most difficult things to remove from a threaded hole, so take your time and twist the handle-- don’t bend.

And we have the perfectly threaded hole.

Our bolt fits in with no problems.

Let’s suppose our widget has served us well for 5 or 10 years, but our bolt hole is getting worn out from putting in and removing our bolt many times or maybe the threads stripped out altogether.  What can we do now?  The easiest answer is to drill our hole out to a larger size and re-thread it.  Let’s take our 3/8-16 hole and expand it out to a 1/2-13.  The procedure is exactly the same, only the sizes change.
For this I will only need:
A 27/64” drill
A 1/2-13 tap
The same tap handle as before
First we look at our handy little chart again, it tells us for a 1/2-13 threaded hole, we need to drill a 27/64” hole.  As a decimal this comes out to approximately .422” so we know we will have a clean hole with none of the left over threads from our (.375” outside diameter) 3/8-16 hole to get in our way.

So we select a 27/64 drill, and re-drill our worn out hole.  To the left you can see the smaller drill we used to make the initial 3/8-16 tapped hole.

Remember when I said that the threads on a tap start at a smaller diameter and gradually get bigger?  Well sometimes you need to thread as far as possible down a hole that doesn’t pass all the way through a part.  In that case, you select a bottoming tap.  You start with a tap with more taper and then finish off with the bottoming tap to fully cut all the threads.  If you look closely you will see the difference, the tap on the left is a bottoming tap.  A bottoming tap is much harder to turn and to get started threading perpendicular to your work piece, so if possible use a tapered tap.

Back at our broken widget, we have drilled the hole with a 27/64” or .422 drill bit.  If you measure a drilled hole it is likely to be several thousandths of an inch larger than the drill.  We can see that this hole measures between .423” and .424”.  It’s nothing to worry about for our purposes, but if your drilled hole is much bigger, the drill bit may need resharpening.

Now we tap the hole with a 1/2-13 tap.  As you tap larger holes,  it is often necessary to go clockwise a fraction of a turn, then backwards.  This breaks the chips formed while cutting the threads.

And here we have our refreshed tapped hole.  Our old bolt is laying next to the new one to show the size difference.

Mr. Bill:

Welcome to TSP Forum, ohio oz!  That was certainly the best first post I've seen in a long time!  Very useful info, very clearly presented.

+1 from me!  Great post - appreciate the time it took to put it together.  Thanks & welcome!

Hare of Caerbannog:
Great post!
I think this may be a candidate for the Save Our Skills pages.

Nicely done! Great info you posted


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