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M3 M4 M5 Captive-Nut Thread Checker
This 84 x 30 x 17 mm bench block holds real steel M3, M4 and M5 hex nuts (5.5, 7 and 8 mm across flats), which slide into side-loaded traps. To check an unmarked bolt, thread it in and see which size it fits. That saves you from trying the bolt in loose nuts over and over. Each station has a 3.4, 4.5 or 5.5 mm clearance bore. A relief channel underneath lets long bolts run out the bottom. Rows of 3, 4 and 5 dimples mark the sizes.
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