Elementary discussion is: SAE has the body diameter in inches or fractions
thereof. The small stuff is numbered, but descriptions of these abound. The
larger numbered ones have thicker bodies. They range from number 12 to 000
or smaller. the real small ones get into jewelers sizes the number 12 is
usually followed by 1/4". These SAE fasteners will have a second number
(the pitch) which is the number of threads per inch. There are coarse and
fine threads per inch. Usual use for fine is to enhance vibration
Metric sizes are in millimeters of body with followed by the pitch. which is
They are not interchangeable.
SAE are generally used in USA and other backward industrial countries. Most
of the rest of the world uses the metric sizes.
The greatest joy you can have is working on a supposedly American car like
Ford or GM etc. and having a mix of SAE and metric fasteners. Or if it was
made in Europe or Asia it might be all metric. This has been a boon for the
tool manufacturers who now have duplicate lines of tools, one for SAE and
one for metric.
For a comparison of the two systems look at any number of web sites. For
openers try http://www.accuratescrew.com/info/MetricSizes.htm#SAE
I don't have anything to contribute to a discussion, but I do have a chart
of drill and tap sizes with their metric equivalents that you might find
useful. Here's the free link:
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