Chapter 4: Rational Numbers: Positive and Negative Fractions Numbers in the Real World

Don't Screw It Up

If you own a car and do any kind of work on it, you learn quickly that one set of wrenches are not enough.  Not only are there special kinds of wrenches, but there are also different measurement systems, SAE and Metric.  SAE stands for the Society of Automotive Engineers who set the standards for that type of measurement in hand tools.  SAE (sometimes referred to as standard) wrenches have sizes based on fractions, such as 1/4, 3/8, 3/4, etc.  Metric wrench sizes are based on whole millimeters, such as 8, 10, 12, 19, etc.

If you have needed one type of wrench when replacing a broken taillight and had the other type, you may have discovered that one of the metric wrenches worked pretty well on the SAE bolt.  We will use a conversion factor to learn which wrenches are somewhat interchangeable.

To convert from SAE to metric, multiply by 25.4.  To convert from metric to SAE, multiply by .039.  For example, a 3/8 wrench (3/8 inch SAE) is converted to a metric size by multiplying by 25.4.

So, a 3/8 SAE wrench is halfway between a 9 and 10 metric wrench.  You probably don't want to use a 10 wrench to take off a 3/8 nut or bolt.  It will probably strip the outsides of the nut or bolt.  However, a 5/8 nut could be taken off by a 16 wrench in most situations.  Verify that this is true.

Construct a table comparing common SAE and Metric sizes.  If you don't own wrench sets, search online for common sizes or visit your local hardware store.

If you do have a set of wrenches, validate your findings by trying out metric wrenches on SAE bolts and vice versa.