Wrench Spanner Set: US or Metric? Which One Should You Buy
If you're looking to buy a wrench spanner set, it's important to differentiate between the two different systems of measurement available: US or metric. The wrench socket sizes will of course vary between sets, sometimes significantly enough where the wrong wrench spanner set will be completely incompatible with the nut or bolt you're attempting to leverage. The following steps will help you determine which set is right for you, so you don't waste any time or energy trying to fix your car with the wrong set of wrenches.
Step 1: Don't Draw Assumptions
Don't assume that just because your car was manufactured in or imported to the United States, that it will by default use the US-based system of measurements (commonly known as the SAE standard: Society of Automotive Engineers). Actually, most cars in the United States use a rather arbitrary combination of metric and SAE fasteners, which can make it frustrating to be a mechanic. You'll need to manually check the individual fasteners you intend to meddle with ahead of time to be sure of the appropriate sizing.
Step 2: Check Your Car
Grab a ruler and go outside to check your car. Hopefully, you already know which nuts and bolts you'll need to loosen to get to the necessary area. With your ruler, measure from one side to the side opposite, making sure that the ruler is held at a right angle to the sides. Line up the outer edges of the bolt or nut as closely as you can and look for the corresponding measurement. Is it closer to falling exactly on a line when measured in millimeters or in fractions of an inch? If all your measurements agree on a single system, you're in luck.
Step 3: When in Doubt...
If you have the money available to do so, it doesn't hurt to pick up a set of both types of wrench sets. As previously mentioned, many cars have fasteners from both the SAE and metric standards, so odds are that eventually you'll need both types of wrench sets anyway. It's far better to have both sets of tools available to you from the beginning than to try to force a wrench onto a bolt that doesn't fit, or to have to make an extra trip to the hardware store.
Step 4: Improvise
Although it's much better for the bolts to use the proper corresponding wrench, there are some quick conversions between the metric and SAE standards that will work in a pinch. If you find yourself with one stubborn SAE bolt when you only have metric wrenches, or vice versa, you can use these approximate conversions from millimeters to inches:8mm - 5/16
13mm - 1/2
14mm - 9/16
15mm - 5/8
19mm - 3/4
21mm - 13/16
Due to the inconsistent construction of cars in the United States, it will likely be to your advantage to have both a metric and an SAE wrench set at hand. However, if you need to choose only one, these steps can help you make sure you're getting the set which will be most useful to you.