1/2 inch vs 13 mm.

[ponytug]

Does that include British Pipe Thread? Man I can stand it when they use that.

Which British pipe thread? Tapered or parallel? BA, BSC, BAF, BSW, BSPP/BSP, BSPT?

https://britishfasteners.com/threads-ba

But, yes, drives me crazy. Add in the hydraulic variations and, I have spent way too much of my life with thread gauges and micrometers. And then I think that there is a special place for the designers who mix metric, SAE, and British threads on the same item.

With Whitworth, I was always grabbing the wrong size wrench, as the wrenches were labeled with the bolt diameter, rather than the head diameter, I.e. a "3/4" diameter thread bolt could have two different whitworth cap sizes, neither of which is 0.75"

British Standard Whitworth - Wikipedia

en.wikipedia.org

I'm not a particular fan of one system over another (e.g. SAE vs metric), but the chaos of the British pipe and fastener systems reminds me of their fondness for their old currency system, with farthings, ha'pennies, pennies, groats, six pence, shillings, half crowns, florins, pounds, guineas, and the whole treasury vs bank notes. They held out against decimalizing their currency for decades because they were of the view that decimals were too complicated. (Yum, yes, of course, "indubitably my good man"...Sure)

Understanding old British money - pounds, shillings and pence

So, yes, probably all of one piece (but not of eight...).

All the best,

Peter

P.S. let's not forget the SAE number, letter, fractional drill and tap complexities.

 

[Hughman]

Already stated above...
1 inch = 25.4 mm

 

[Paddy]

I've always used these interchangeably, although the metric is slightly bigger.
(+- 2%) Yet 1/2" is too small for the nuts on the bar for my Echo saws. Yesterday I was tightening the tailgate latch on my pickup and found that 1/2 was too small for that, also. Apparently my 1/2 inch sockets only identify as metric.

the 5/16 bolt will go into the threads of an 8mm course threaded hole

 

[bmaverick]

4 / 25.4 = 0.1575"

A hair bigger than 5/32" but now your talking hex-keys most likely if you are working with stuff that small. And with hex heys....about the only ones I will interchange are 8 and 5/16. Cause aint much worse than a rounded out hex-key or bolt

4mm are small hex head screws on my GMC truck. No hex key will take those off.