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I have a metric thread pitch gauge that came in a tap and die set, it has pitches like $0.75$, $0.8$, $1.25$, etc. which is the distance between each thread. However there are two gauges that say $27$ and $28$ - I thought maybe it means $0.28\text{ mm}$ or possibly $0.28\text{ inches}$ if they threw in some non-metric ones, but it is neither of these (it's about $0.9\text{ mm}$) what are these?

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28 is a standard Threads Per Inch dimension for 1/4 and 1/2 inch screws.

$\frac{25.4 \,\text{[mm/Inch]}}{28\, \text{[Threads/Inch]}} = 0.9 \,\text{[mm metric pitch]}$

The 27 seems strange to me. But I usually use metric bolts.

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    $\begingroup$ 27 TPI is used in pipe threads $\endgroup$ – Dan Feb 25 '15 at 20:11
  • $\begingroup$ Thank you. This was confusing, are all non-metric thread measurements in threads-per-unit rather than units-per-thread? $\endgroup$ – jhabbott Feb 26 '15 at 14:03
  • $\begingroup$ I couldn't say, I'm often suprised by the imperial system:) I think TPI is rather common. $\endgroup$ – Walter Feb 27 '15 at 9:08
  • $\begingroup$ Yes, in inch system threads the usual unit of measure is threads per inch, not thread pitch in inches. 1/4" DIA and 28 TPI is the US standard fine thread, but the standard course thread 1/4-20 is more common. $\endgroup$ – Ethan48 Jun 16 '16 at 13:16
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As mentioned these are 'teeth per inch' threads. they find their way onto metric thread gauges because BSP threads are still the standard in many countries for tings like gas fittings.

This is partly because of legacy issues where it would simply be too much hassle to change over millions of fittings on things like industrial gas bottles and the associated regulators and fittings as opposed to nuts and bolts which have a much higher turnover and in many cases will allow a straight swap for the nearest equivalent metric size.

Another advantage is that discourages bodges using standard nuts and bolts for gas fittings which have specific safety requirements.

Also BSP type fittings are generally so specialist that there is little advantage in standardising to metric as they will only ever be used to mate to each other.

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