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Can a threaded stud of mild steel, size 5/8"-11 over a threaded portion of 1.25" stretch so much over time that it reads 5/8"-13?

Unthreaded portion of the stud reads .625-630" in diameter, threaded area is in the .610-.605" range, there is visible thread blunting/wear, more progressive the closer you get to the end of the stud.

I know the M16 x 2.00 is sometimes miss-read as 5/8-13, but my M16 x 2.00 die does not chase over the stud, and my tap does not work through the stud's accompanying nut.

I know the machine it comes from is old, and though I've never seen it, I've heard of proprietary threads, so before I go crazy looking for/trying to make a weird sized tool, I figured I'd ask.

So has anyone ever seen a thread stretched so far it reads a different pitch, yet visibly, it doesn't look so deformed?

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    $\begingroup$ could you post a picture with the stud preferably next to a reference item. $\endgroup$ – NMech Oct 29 '20 at 19:36
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    $\begingroup$ One might stretch the stud in an unsupported section, but the area inside the Nut wouldn't deform the same way . Can you run the nut up and down the whole length of thread? $\endgroup$ – Jonathan R Swift Oct 30 '20 at 8:08
  • $\begingroup$ @JonathanRSwift The nut is flanged and fairly large in height, probably 1.25" or so, it goes on about 1/2-5/8", flanged side down, before it gets to tight to turn by hand. That makes sense about the deformation of the stud, any part inside the nut wouldn't deform so much as the flange of the nut, and the rest of the stud take the load. So due to necking of the root of the thread, the most deformed area should be right under where the flange of the nut ends, given the nut sat at that particular point in the thread for significant time. $\endgroup$ – Corey Oct 30 '20 at 12:10
  • $\begingroup$ Great - so this supports the theory that the thread has stretched. Does a 5/8"-11 tap fit through the nut? $\endgroup$ – Jonathan R Swift Oct 30 '20 at 12:14
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That looks like roughly 15% elongation. I would expect it to start necking by then, especially when starting with essentially a notched ( threaded) bar. Also , with overall elongation of 15% , most of the elongation would be in the thread roots ,having a higher stress because of the smaller diameter. Do the thread root contours look correct ?

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