Metric Bolts

The International Standards Organization (ISO) has designated the metric system as the world standard of measurement.

As far back as the early 1970's when Ford introduced the 2300cc, 4-cylinder engine in the Pinto, the use of metric fasteners have become more prevalent in domestic vehicles. Probably the majority of domestic vehicles on the road today have more metric fasteners than the inch-size (SAE) type and almost all the fasteners on vehicles currently being produced are metric.

The mixture of metric and SAE fasteners on the same vehicle means that you have to be very careful when removing bolts to note their locations and to keep metric nuts and bolts together. At first glance, metric fasteners may appear to be the same size as their SAE counterparts, but they're not. While the size may be very close, the pitch of the threads (distance between threads) is different. It is possible to start a metric bolt into a hole with SAE threads and run it down several turns before it binds. Any further tightening will strip the threads. The opposite could occur also; a nut could be run all the way down and be too loose to provide sufficient strength.

Fortunately, metric bolts are marked differently than SAE bolts. An ISO metric bolt larger than 6 mm in diameter has either "ISO M" or "M" embossed on top of the head. In addition, most metric bolts are identified by a number stamped on the bolt head, such as 4.6, 5.8 or 10.9. The number has nothing to do with the size, but does indicate the relative strength of the bolt. The higher the number, the stronger the bolt. Some metric nuts are also marked with a single-digit number to indicate the strength, and some may have the M and strength grade embossed on the flats of the hex.

Metric nuts with an ISO thread are marked on one face of the hex flats with the strength grade (4, 5, 6, 8, 12, and 14). Some nuts with a 4, 5 or 6 strength grade may or may not be marked.

A clock face system is used as an alternate means of strength grade designation. The external chamfers or faces of the nut are marked with a dash at the appropriate hour mark corresponding to the relative strength grade. One dot indicates the 12 o'clock position and, if the grade is above 12, 2 dots identify 12 o'clock.

The size of a metric fastener is also identified differently than an SAE fastener. A metric fastener could be designated M12 x 2, for example. This means that the major diameter of the threads is 12 mm and that the thread pitch is 2 mm (there are 2 mm between threads). Most importantly, metric threads are not classed by number of threads per inch, but by the distance between the threads, and the distance between threads does not exactly correspond to number of threads per inch (2 mm between threads is about 12.7 threads per inch).

The 25 standard metric diameter and pitch combinations are shown here. The first number in each size is the nominal or root (minor) diameter (mm) and the second number is the thread pitch (mm).

NOTE: Remember that the nominal bolt diameter is the measurement of the bolt diameter as taken from the bottom of the threads NOT the top (which would be major diameter).