They say that if we were supposed to measure in metric, King Henry’s foot would have been 30.48 centimeters long instead of 12 inches. Unfortunately, we Americans are about the last ones using inch measurements. We don’t even use it consistently. Bottles are often described in ounces while that same bottle’s cap is almost always sized in millimeters.

For machinery, it can be even more confusing. Many plants have both U.S. (inch) and imported (metric) machinery on the packaging floor. In some cases, a single machine may mix both standards. For example, a U.S. system may incorporate a sub-system that uses metric measurements.

My particular pet peeve is the use of metric tools on inch fasteners and vice versa. For some sizes, they almost work. The metric wrench will be a bit loose but will remove the inch bolt as long as it is not too tight. If it is too tight, it will slip. This can cause a personal hazard such as cracked knuckles. In some cases, it will round the hex corners making it difficult or impossible to remove even with the proper wrench.

Recessed hex setscrews can be particularly difficult to remove once this has happened.

Training will help avoid these problems. One technique is signs on machines identifying them as inch or metric. Proper tools must be provided and clearly identified. Some mechanics will have a tool drawer with a number of Allen style wrenches, in all sizes and states of wear, jumbled together. This makes finding the proper wrench difficult and time consuming. Allen style wrenches are available with a blue coating to clearly distinguish them from the standard black, inch, wrench. These should be purchased for all mechanics and all black metric wrenches collected and discarded. While you are at it, make sure the inch wrenches are in good repair as well.

End, box and socket wrenches can be color coded with paint or tape, blue for metric and red for inch.

Improper fastener use is another challenge. A metric bolt will be misplaced and an inch bolt substituted. It doesn’t really fit but the mechanic will attempt to force it. The worst case is when they are able to. The bolt appears to be tight but will fail at the least opportune moment. Otherwise, they will bollix the thread then may drill it out and rethread it in an inch size. This is a guarantee of future problems.

Inch and metric thread gauges should be in every mechanic’s toolkit to aid in properly identifying fasteners when in doubt.

In the end, it comes down to awareness and training. It’s not that hard to keep the inch/metric mix straight. It does require that people pay attention.  F&BP

John Henry, Certified Packaging Professional (CPP), is renowned as the Changeover Wizard. His company,, specializes in improving line efficiencies for packagers by reducing downtime. Contact John at or 787-550-9650.