How to Match Screw Threads

How to know if two bolts have the same thread.

 Well, I had to match up some screw threads with some nuts today, and it reminded me of a little trick I learned long ago. With both fine and coarse threads in fractional sized bolts, and various pitches in metric bolts, it's sometimes hard to know if they will screw into the same hole or not:

Hold the first bolt horizontally in front of you.  place the threads of the second bolt on top of the first  bolt.  If they are the same thread, the threads will mesh.  If not, they will sit on the top (points) of the threads.

Make sure the bolts are of the same diameter as some bolts have the same "pitch" but are different diameter.

Now a little background on screw threads and pitch.

  The actual geometry of screw threads is actually fairly complex. In their simplest terms, they are defined  by two things:

1. Their basic diameter-- 1/2 inch, 5/16 inch, etc. for the English system of measurement, and 5 millimeter, 10 millimeter, 12 millimeter etc. for metric diameter fasteners.

2. Their pitch-- Thread pitch might be confusing, but all it really is is how many threads there are in a certain length. 

In the English System, pitch is defined as "threads per inch". For example, half-inch diameter bolts can have a thread pitch of 13-- thirteen threads per inch of running length of the screw. You will find it designated like this:

1/2-13, or 1/2 inch diameter, 13 threads per inch.

 In the Metric System they define how many millimeters per thread... for example, a bolt with a 1.5mm pitch has threads that are 1.5 millimeters wide, or a little larger than 1/16th inch.

A metric fastener would be designated something like this:

 8x1.5mm, or 8 millimeter diameter with 1.5 millimeters per screw thread.

The English system is used primarily in the United States and in other English-speaking countries, though it has been pretty much replaced by the metric system in all but the US. It is based on even fractional numbers starting with 1/4 inch up to 7/8 inch in 1/16 inch increments, and larger sizes in 1/4 inch or larger increments-- 1", 1 1/4", 1 1/2", 2", etc.

The Metric system is the most common system, and is used world-wide. It is a decimal-based system, and though a little challenging at first, is actually easier to use than the English or fractional system once you get used to it.

One way to relate to the metric system is by comparing it with the decimal monetary system used by the dollar used by the US and several other countries, like Canada, New Zealand, and Australia. This currency is decimal based-- 10 pennies equal a dime, 10 dimes equal a dollar, etc.

Hope this helps...

Smitty