I have to replace a nut on an old sliding door that dates back at least 50 years. The nut fits a 7/16 spanner, but that's all I know about it. Unfortunately I can't take the nut off and take it to my local hardware shop because the door is in continual use and would have to be wedged in place when I take the nut off. So I have to try and work out what the thread is so I can buy a replacement nut.
From Googling it seems that if a 7/16 spanner fits the nut is (probably?) either 3/16 Whitworth or 1/4 BSF. And I can probably estimate the bolt diameter to enough accuracy to tell the difference.
My question is: does the above make sense. If I hold a ruler up to the bolt and it looks nearer to 1/4" than 3/16" do I just go out and buy a 1/4" BSF nut? Or a 3/16" Whitworth nut if the bolt looks nearer to 3/16" than 1/4"?
Not that buying the wrong nut will bankrupt me, but it's a long drive (through the snow!) to the hardware shop :-)