INCH SIZES
"It almost fits." Why an M8 is not a 5/16"
Inch and metric bolts sometimes look alike. Here is how to avoid a bolt that shakes loose.
The most dangerous bolt is not the bolt that does not fit, but the bolt that almost fits. Take a look:
| Inch | Diameter | Metric | Diameter |
|---|---|---|---|
| 5/16"-18 UNC | 7.94 mm | M8 × 1,25 | 8.00 mm |
| 3/8"-16 UNC | 9.53 mm | M10 × 1,5 | 10.00 mm |
| 1/2"-13 UNC | 12.70 mm | M12 × 1,75 | 12.00 mm |
An M8 turns a few times into a 5/16" hole and then jams. Or worse: it seems tight, but the thread is damaged. Under the vibrations in the field it shakes loose.
How to be sure
Measure the diameter with a calliper and count the threads over one inch (25.4 mm). If in doubt, lay the bolt next to a calliper, take a photo and send it to us.
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