The Right Way, The Wrong Way and The American Way

Battlebots Trophy -- A nut with a 5 inch diameter hole with 4 threads per inch

The Battlebots Trophy — A nut with a 5 inch diameter hole and 4 threads per inch

By The Metric Maven

GAO Report Edition

Fasteners are ubiquitous. In physical design, they are everywhere. The image to the left is the current Battlebots Trophy, a giant nut. As of 2016, this nut celebrates US obsolescence with its archaic 5 inch diameter hole and four threads per inch.  The 1978 GAO report has an interesting chapter on fasteners, what happened, and what didn’t. The first sentence of the fastener chapter reads:

The U.S. fastener industry which was originally opposed to metrication, began conversion efforts in 1970 in order to maintain its markets.

The industry found that in the 1960s their major customers were moving toward the metric system. One would think the US fastener manufacturers would have been in favor of metric by 1978, but the story is more complicated than expected and perhaps too American. The Report has a nice description of a fastener:

A fastener is anything which holds two things together. Nuts, bolts, screws, rivets, cotter pins, and nails are a few examples. (See following page.) Of these, the United States produces approximately two million different types. Fasteners can hold together a vast number of items. For example, a telephone is held together with about 70 fasteners. Jumbo jets contain millions; and for one model, fasteners costs represent about 10 percent of the plane’s total cost. In short, much of the nearly $2 trillions U.S. economy is held together by the $2 billion fastener industry.

The report notes that a considerable increase in the use of metric fasteners is taking place in the US. The domestic fastener industry was also under pressure from imported Olde English fasteners. At the end of the 1960s, no US engineering standard for metric fasteners existed, but an international standard did. US industry representatives claimed that the international standard had too many sizes and thread types. The values of these sizes did not follow a logical pattern it was alleged.  If the US fastener industry was going to become metric, it was argued that the US should create a new fastener system that was:

….as perfect as possible. Also, the industry did not want to give a competitive advantage to foreign producers of metric fasteners. It was felt that the foreign producers would gain an advantage if the U.S. industry merely accepted the existing international  standard for metric fasteners in its entirety.

Yes, we in the US were going to produce a “more perfect fastener” or perhaps even a perfect fastener, and in January of 1971 the report “A Study To Develop An Optimum Metric Fastener System” was released by the Industrial Fasteners Institute. The study was presented to the ten largest corporations in the US as well as the National Bureau of Standards (NBS), and technical bodies in Canada. The selected group was unanimous in its view that a detailed study should be undertaken. The GAO Report states:

The Committee’s ultimate objective was to design a metric fastener system which would be so attractive technically and economically that it would become the single internationally accepted system of threaded fasteners. (7-5)

An unshakable US faith in technical Darwinism, coupled with the belief the US would create the fittest fastener meme propelled this new study. The Special Committee published its results in 1973. It recommended a fastener system with 25 sizes and a single thread type. The first metric fastener standard based on these recommendations was released in 1974. Before the standard was completed, the new system was encountering international resistance. Britain and German standards representatives released a paper called “Why Should the International Standards Organization System for Metric Fastener Threads be Changed?” It argued that the costs and confusion were unwarranted, “the technical advantages were minimal, and the system could hardly be called “optimum.” There were complaints of protectionism and everyone having to start all over again. (7-6).

The discussions continued from 1973 to 1977 as the ISO negotiated with its US members. The US representatives finally backed off from the proposed changes to the international standard. The US standard became essentially the same as the preferred series of the ISO standard.

There was controversy about the strength grade of fasteners in the 6 to 18 millimeter range. Europeans used an international strength grade of 8.8. It has a strength capacity of 116 000 pounds per square inch. The comparable US SAE was grade 5, which has a strength of 120 000 pounds per square inch. This is about a three percent difference. It was recommended the next higher grade 9.8 be used. This fastener has a strength of about 130 500 PSI.

The Europeans went along with the proposed change, but only the US automotive industry adopted the higher grade. US farm equipment, Canadian and European manufacturers decided to use 8.8 for their threaded fasteners. The unavailability of fasteners that met the US requirement caused concern that an 8.8 fastener could be interchanged for a 9.8 version during a repair. If 9.8 was not available, it would be necessary to use 10.9, which requires an alloy steel.


The report next focused on the head sizes for the fasteners:

A major problem arose during the attempt to reach agreement on the hexagon head size for three fasteners. This was probably the most hotly debated and difficult issue considered during the 1977 ISO meetings. The schedule below shows the head sizes wanted by the United States, those used in Europe, and those agreed to at the meetings.


The Optimum Metric Fastener System study had shown that the head size for a number of fasteners was unnecessarily large. International standard sizes were widely used in Europe, but the European representatives had in 1975 agreed to reduce the head size 1 millimeter on each of the three sizes. The U.S. representatives agreed to the compromise sizes in the earlier meetings, but in 1977 returned to the demand for a smaller head for the 10-millimeter fastener size.

The Europeans would not approve an inclusion of a 15 mm head and the US would not compromise. The official standard became 10, 12 and 14 millimeter diameter fasteners with 16, 18 and 21 millimeter heads respectively. The US would use these and the 15 mm head. It became possible that several head sizes might be used for these three fastener sizes. The Report noted:

Head sizes (like strength grades) are an example of an international standard which is formally agreed to on paper but not uniformly adhered to in practice. (7-8)

The European view was that the benefits of the changes to the new system did not justify the expenses involved. The fastener standard is voluntary, and the US could do whatever it wanted. This impasse could leave US fastener manufacturers holding the bag. The GAO report states:

An official of one company told me he had stuck his neck out and stocked six metric sizes in 24 lengths. The stock included the 6.3-millimeter fastener which was one of the U.S.-proposed sizes that did not gain international acceptance. This size was being used by a major automobile manufacturer in its 1977 and 1978 models. However, the automobile manufacturer has dropped it for future models.

It was noted that maintaining Ye Olde English and metric fasteners in the US could cause considerable difficulty:

It is virtually impossible to visually identify some sizes of customary-threaded fasteners from similar-size metric fasteners. It is possible to mismatch 36 combinations of customary- and metric-threaded fasteners. The result could be either stripping during assembly or full assembly with 25- to 60-percent loss in load capacity. Thus, the accidental mismatch of fasteners could result in fastener failures.

This is a very good argument for a quick metric switch-over, with an M-day, and no “transition period,” rather than waiting for the magic of the Metric Philosophers technical Darwinism to accomplish this task over an undefined period.

On April 25, 2014 (2014-04-25), Joe Greenslade of the Industrial Fasteners Institute gave a presentation titled “Metric Fastener Standards Transition”  His view is that one metric fastener system should be used throughout the world. Mr. Greenslade calls the US attempt to create an “Optimum Metric Fastener System” a “misguided move!” He claims that from 1975 to 2013 there has been a slow but gradually accelerating adoption of metric designs.

Greenslade identifies three different fastener systems, ISO (International), DIN (German) and ANSI/ASME/ASME/SAE (US). He sees the US Optimum Metric Fastener System (OMFS) as a misguided philosophy of “since we must change we will do it better than you Europeans do.”  The OMFS attempted to eliminate fine threads, this “simplification” was rejected. The US introduction the M6.3 X 1 fastener simply because we wanted a metric version of a 1/4-20 Ye Olde English fastener, rather than using a standard M6 x 1.0 was rejected. The introduction of a new thread gauge was not accepted. The US wanted to replace the hex head with a new spline head, but that was also rebuffed. The changing of hex sizes (head sizes) by 1 millimeter on M10, M12, M14, and M22 is still causing confusion to this day. The US has finally withdrawn its proposed “optimum” metric standard.

The two metric standards that remain are ISO and DIN. DIN is very, very close to the ISO standard. They are 99.99% interchangeable, and 90% identical. The German DIN standard is to be replaced with the ISO standard. When US customers now ask for ISO they are often told “we do not stock any ISO — only DIN.” Greenslade indicates that a search for dual DIN/ISO designations on existing drawings and parts lists should be undertaken, and in these instances they should be edited so that only the ISO number is used. He also suggests that all new product design drawing designations be only ISO. Greenslade offers numerous examples of this existing redundancy “out in the wild.” The long term objective for the USA should be to use ISO and forget the past.


baskin-robbinsThe US introduction of an “optimum” standard in the 1970s has the fingerprints of American hubris all over it. Rather than finally bring some order to the chaos that is side by side Ye Olde English fasteners and metric, by eliminating the “custom”-ary versions, and using ISO metric exclusively, we instead opted to show everyone “how to do it better.” History has not judged us favorably, and the exercise in imposing a US metric “standard” on the world continues to cause discord and confusion to this day. As has been said many times “the great thing about US standards is there are so many to choose from.”

Related essays:

Without Metric Threads We’re Screwed

A Hole in The Screw Head

The Metric Maven has published a book titled The Dimensions of The Cosmos. It examines the basic quantities of the world from yocto to Yotta with a mixture of scientific anecdotes and may be purchased here.