I was talking to Dan (from ToolsinAction) the other day, when he brought up the Thread Checker. I know all about the Thread Checker!, I told him, as I remembered posting about it as part of a larger screw and bolt identification tool post from a long time ago.
The Thread Checker is almost self-explanatory. It’s composed of individual thread gauges that are looped onto a length of steel wire, with each gauge featuring male and female threaded sections.
It’s a go-no-go gauge, where you attempt to screw an unidentified bolt into the female threaded section of a gauge, or a nut onto the threaded stud. If the hardware fits, then it’s a match for the size marked on the gauge. If not, try again.
S&W Manufacturing, the USA company that makes these under the Thread Checker brand, offers different gauge sets. There’s a 26-size inch and metric set SWTC-26, shown above, which has inch sizes from 6-32 thru 1/2-20, and metric sizes from M4x0.7 to M12x1.75.
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There is also a 23-size inch-only set (SWTC-S21), and a 21-size metric set (SWTC-M21). You can also buy a set that comes with both.