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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 :-)

  • I would drive there once, and buy both. – Mark Mar 2 '18 at 14:39
  • @Mark, yes :-) But that assumes it isn't even more complicated than I thought. Ed Beal now tells me there are different thread pitches as well as different types of nut! I kind of suspected it wouldn't be simple ... – John Rennie Mar 2 '18 at 14:41
  • Whats the chance of it being metric? Or even a "gauged" screw, for example 12, 10, and such... – Jack Mar 2 '18 at 16:02
  • @Jack very slim. It dates from well before metric bolts were common in the UK, and a 7/16 spanner is a perfect fit. – John Rennie Mar 2 '18 at 16:03
  • Were 12 gauge screws or the 10 gauge used back then? – Jack Mar 2 '18 at 16:04
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A few alternative approaches that don't require guessing:

  • If the nut fastens to a removable bolt rather than a bracket with a permanent threaded rod, don't bother trying to match the nut. Replace both the bolt and nut with something about the same size that fits the existing holes.

  • If there is a removable bolt, but it also threads through other hardware, temporarily remove the bolt and nut and measure the size using the bolt.

  • If there is not a removable bolt, temporarily remove the nut and measure the size and thread of the exposed portion of the "bolt". Put the nut back on until you buy the replacement. If you have a thread gauge (or there are cheap ones you can buy), that will make it easier to get an accurate measurement on either the bolt or nut.

  • If you don't have a thread gauge and measurement is difficult, temporarily remove the nut. Lubricate the nut and threads, then make a quick "casting" of the thread with hot melt glue or other material, including enough material to grab to unscrew the casting. Unscrew the casting and put the nut back on the door. Then take the casting to the hardware store to find a matching nut.

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If it's 1/4" the nut would be 1/4-20 or 1/4-28 with 1/4-20 being more common. The next size up would be 5/16-18 , or 5/16-24 and next 3/8-16 & 3/8-24 these would be the sizes close to or larger of the size described. If you can measure the bolt length or at least 1/4" and count the threads in the 1/4" and multiply by 4 should provide the correct pitch or be close enough that you won't need to purchase more than you need to get the right size.

  • It certainly can't be a 5/16 or 3/8, there wouldn't be enough meat on the nut to allow a 7/16 spanner. – Harper Mar 2 '18 at 17:18
  • These are US gauges. Over in the UK they use either BSF or Whitworth, the latter having a strong resemblance to US NC. – Harper Mar 2 '18 at 17:28
  • Thanks Ed. As Harper says, this is in the UK. Sorry, I should have made that clear in the question. – John Rennie Mar 3 '18 at 5:46

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