About Bolts And Screws, A Let’s Build Resource

Bolts are threaded fasteners usually intended to be used with a nut. Screws are everything else except studs, which are full threaded rods designed to threaded into a tapped hole and then have a nut attached.

Bolt terminology.


Some common bolts


Another screw and bolt table.

Identification Charts for Different Types of Fasteners – Head styles, Bolt and Screw, Drive, Washer and Nut types

Standard UN bolt sizes.


Standard metric Bolt sizes.


More screws and bolts.


Another screw and bolt chart.


Threads are usually either Unified thread system or Metric threads.

A UN screw or bolt will be called out with a number which will correspond to wire size and a threads/in say 10-32, or a decimal or fractional dimension followed by the threads/in 5/8-16.  That callout will be considered coarse threads or if UNF follows the callout, fine threads.

Here’s a table of thread sizes.


In addition to fine threads there are also thread callouts for special needs such as extra fine, UNEF, special, UNS and UNJ which is used for aerospace application where a tight thread fit is desired.

A quick primer on bolt preload.

Relative sizes of common thread pitches.


Note that a .8 mm thread pitch and 32 threads/in are very close together.  for m5 and #8 screws make sure that you use just one in your design. Trust me on this, from 10 years and thousands of people asking for longer screws on cabinet door handles, you will mix the two up.

Some standard UN machine screw sizes.


Some standard metric machine screw sizes.


Larger than M8 or 1/4-20 and you probably want to use socket heads in your design. In fact I would say larger than M5 or #10 and you switch to socket heads.

Washer sizes.




Preload is why bolts are available in different grades and strengths and why bolted joints have a torque rating applied to them.  Preload is also why locking nuts or washers are not used on a bolted joint.

In addition to screws for prethreaded holes there are screws designed for cutting their own threads.


One problem that’s been around forever is how do you keep people from unscrewing a bunch of screws and opening something you don’t want opened for various reasons. Enter tamper proof screws.


Purchasing fasteners is fairly easy through McMaster Carr or specialty house like this.


This is my local company.


Better hardware stores will have a range of bolts and screws, though the better stores are being replaced with big box stores who’s selection tends to be more home owner oriented rather than engineering or project oriented. Still there are some good stores out there I the middle of industrial areas. The Fastenal industrial hardware chain has a goo

When you are designing, you want to stay with standard fasteners as much as possible. Unfortunately, all too frequently, that’s not possible. that’s when you go to the custom screw manufacturers like Accurate screw machine.


Threaded fasteners can be a headache, but if you know what you are doing, stay as much as possible in the common sizes and  know how to buy custom fasteners when you need to you can make incredible things.

The Let’s Build Series.




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