A Primer On Threads

Threads are confusing, there’s no getting around it. But there are some aspects of threads that are easier to understand and some that are harder. For our purposes, we will mostly stick with traditionally American threads but we will quickly mention metric threads at the end.

There are a lot of different kinds of threads. Some of these include (but are not limited to): Unified screw threads, miniature threads, Interference-fit threads, M profile metric threads, MJ profile metric threads, Acme screw threads, Buttress threads, British buttress threads, British ISO metric threads, Whitworth and fine form threads, Taper pipe threads, straight pipe threads, dryseal straight pipe thread, dryseal tapered pipe threads, hose coupling threads, British pipe threads, fire hose threads and more.

The threads consumers deal with mostly are the unified screw threads, along with straight and tapered pipe threads. The others are for specialized uses. As an example, if you ever see a faucet stem taken apart, there is a wide form funny looking thread that generally has very few threads per inch…this is most often an acme thread and is used because a few turns travel a long distance.

Unified screw threads come in numbered sizes (for small threads) and as they get larger, as a size expressed in inches. These threads are most easily understood when a few terms are explained. The drawing below shows both an external (male) thread and an internal (female) thread. Both have characteristics in common. The major diameter is the diameter of an imaginery cylinder that would bound the crests of an external thread or the roots of a straight internal thread…AND NOW I’VE LOST YOU!…So let me state this differently, somewhat inaccurately, but for our purposes simply…The major diameter is the largest diameter of the thread and is the diameter that defines the thread’s size. A ½” thread will be just about ½” in diameter when measured. You cannot measure a ½” internal thread easily because the major diameter is not readily visible or measureable. This necessitates using a thread gage to determine the size of an internal or female thread. The minor diameter is the smallest diameter and the pitch diameter is the theoretical midpoint between them. This holds true for both external and internal (male and female) threads.

In the end, the easiest thing to do to determine a thread size is to take the thread to a hardware store and ask for help.

An External Thread...also called a male thread
	Major Diameter
	Pitch Diameter
	Minor Diameter

An Internal Thread...also called a female thread or a tapped hole
	Major Diameter
	Pitch Diameter
	Minor Diameter