“Fastener Wars”: Metric and SAE Fasteners in the Auto Industry

We probably take fasteners—nuts and bolts—for granted until we reach into our toolbox for the right socket wrench for the engine bolt we want to remove. Do we need an SAE socket or a metric one? If we pick the wrong one, it may not work as expected. In fact, if we pick the wrong one, it may just slip around the nut or the bolt head without getting any traction at all. We also run the chance of using say an SAE bolt as a replacement with a metric-threaded nut or hole—or vice-versa.

Why is there a difference? Why have both SAE and metric nuts and bolts been used on American cars over the years? And what’s the situation now and in the future?

Some History

ISO Metric and SAE (Society of Automotive Engineers) are both industry standards for nuts and bolts. These standards govern not only the nut head or bolt size, but also the configuration—diameter and pitch—of their threads. Both standards have hexagonal bolt heads and nuts, but the SAE standard is defined using inches, while metric is based on millimeters. SAE was based on earlier British and American standards. Although the term SAE is still in common use, the standard itself has been incorporated into the US National Unified Thread Standard (UTS), where it is known as UNF (Unified National Fine).

After the Second World War, while the Brits and the Yanks were defining and refining inch-based standards, the International Standards Organization (ISO) was creating ISO metric screw thread standards. The metric standards were readily adopted by European and Japanese car makers, while U.S. and British car manufacturers initially stuck with inches.

Of course, we know what happened in the latter half of the 20th century, as foreign car companies—German, Japanese and Korean in particular—came to dominate the U.S. car market. And with the flood of imported cars, metric socket sets and wrenches became a necessary part of every car mechanic’s tool box.

The Move to Metric

fasteners auto industryBut it’s no longer as simple as saying “foreign car = metric; domestic car = SAE.” The car manufacturers are all global companies now, with global design teams and supply chains, as well as far-reaching alliances with nominally competing manufacturers. For example, GM has imported cars built by Toyota, Isuzu and Suzuki and sold them under their own Geo brand name. At the same time, Ford, Chrysler, and GM had subsidiary companies in Europe and Australia which designed and manufactured cars for their markets. In many cases, the U.S. parent would import and sell rebadged versions of those cars in the States, or they would build cars in their U.S. plants using overseas-designed or manufactured powertrains. In some cases, this use of outsourced components or designs also extended to competitors such as Toyota, Honda, BMW, and others.

So Back to Fasteners

What did all this mean for SAE and metric fasteners? Well, starting from a time when domestic manufactured cars used only SAE fasteners and foreign cars used metric ones, we’ve seen a steady move towards metric. This is especially true for those globally-sourced powertrain components—engines, transmissions, drive shafts, differentials, etc.—that will be used in both foreign and domestic factories.

Today and Going Forward

Today, the majority of fasteners in new cars sold on the U.S. car market will be metric—regardless of where the car is actually manufactured. So while you’re working on your Dad’s classic Chrysler, Ford, or Buick, you’ll probably need both SAE and metric tools in your bag. For your late-model Detroit ride, you’ll be relying on your metric socket set and wrenches for the most part.

Even if you are well equipped with both metric and SAE tools and fasteners, with the increasing use of robotics in car manufacturing, you’ll also need a Torx® or star-shaped socket set as well! Some conspiracy-minded folks are convinced that it’s all part of the auto industry’s plan to squeeze out do-it-yourself auto enthusiasts. But that’s a discussion for another time!

What types of fasteners do you find in your car?

About the author

Mr. Metric is the spokesperson for www.mrmetric.com, an online store for metric fasteners. Get $6.00 flat shipping with no minimum orders, plus advice and best practices for using metric fasteners in any application.

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