Understanding Metric Fasteners - Fastener Mart

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Fastener Tech Data | Understanding Series Introduction

The size of a metric screw or bolt is specified as diameter, pitch and length, in millimeters (millimeters is abbreviated "mm"). For nuts, size will appear as diameter and pitch. There is an exception though—if pitch isn't specified, the fastener is coarse threaded. Consider this size…

M8-1.0 x 20

Here's what the description means…

M = Metric thread designation

8 = Nominal diameter, in millimeters

1.0 = Pitch (distance from thread to thread), in millimeters

20 = Length, in millimeters (see below)

This example illustrates how a coarse threaded screw may appear…

M12 x 25 = 12 mm diameter, coarse thread is assumed (1.75 mm), 25 mm long

M12 is a matching coarse threaded nut

(To avoid confusion, we always include the thread pitch for both coarse and fine metric fasteners.)

If you normally work with inch fasteners, pitch can be a little confusing—pitch is the distance between adjacent threads. In the example above, a pitch of 1.0 means that from one thread to the next thread, the distance is 1.0 millimeter. Inch fasteners are described as threads per inch, which is sometimes mistakenly called pitch. If you need to know the threads per inch of a metric fastener, first convert pitch from millimeters to inches (multiply by 0.03937), then divide 1 by that number. Or, use the table below…

Metric fasteners are usually classified as either coarse or fine, referring to their thread pitch, although some sizes are available in an extra fine thread. Coarse metric fasteners, when compared to standard "inch" fasteners (those commonly used in North America), tend to be less coarse—their threads are more closely spaced—for a comparable size. Fine metric fasteners can be more or less fine, depending on size. Coarse is the most common metric thread and, as we've mentioned, coarse is assumed when pitch isn't specified. See the table below…

The threads of an M14-2.0 screw, which is coarse, are spaced 0.079 inches (2.0 mm). A 9/16-12 screw, which is comparable in diameter and is also coarse, has its threads spaced 0.083 inches, which are farther apart, so there will be fewer threads per unit of measure (we used an inch in the table above). The same goes for a coarse M3.5-0.6 and a 6-32 screw—the threads of the 6-32 are more widely spaced so, again, it will have fewer threads for the same length. (M3.5-0.6 and M3.5-.6 are exactly the same—the "0" is included to make sure that the decimal point isn't missed.)

When you look at fine M14-1.5 and 9/16-18 fasteners, however, the 9/16-18 is finer—it has more threads per inch. But the M3.5-0.35 has close to double the number of threads per inch of a 6-40, so its threads are very closely spaced (0.014 inches, or 0.35 mm).

The table below compares various metric sizes to the closest inch equivalent…

Length is measured the same as inch fasteners, except in millimeters. For button, cheese, hex, pan, socket and low socket heads, measure length from under the head. For flat heads, length includes head height. Measure oval heads from the largest diameter point—where the dome top meets the conical bearing surface—to the tip.

Metric fasteners are commonly manufactured to meet standards set by DIN - German Institute for Standardization or ISO - International Organization for Standardization. Consequently, they are referred to by the applicable standard number. You may also see JIS - Japanese Industrial Standard and ANSI - American National Standards Institute specified. Following is a table of common DIN and ISO numbers and a brief description of the fastener…

Note: If you know the DIN or ISO number of the fastener you're looking for, you can use our search function to locate it quickly.

Remember these tips about metric fasteners…

Below is a partial list of metric sizes (diameter-pitch) Fastener Mart offers (be sure to search our store for other sizes)…

The differences between inch and metric thread sizes are summarized in these tables…

Some metric fasteners are available in different strength grades, which is referred to as "property class" or just "class." See Understanding Fastener Grades and Classes for a comparison between metric and inch series strength grades. Also, visit Understanding Fastener Materials and Understanding Fastener Platings and Finishes for more information relating to metric fasteners.

Fastener Tech Data | Understanding Series Introduction

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