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I lost one of the screws of my grandpa's rocking chair. I live in Germany and cannot figure out which kind of nefarious imperial screw this is. According to my measurements, the diameter is exactly 4.5 mm and the length (head included) 30 mm. However, this is not M4.5. I measured a thread pitch of 1.04 mm which arguably results in a thread density of 24 TPI (threads per inch). I assumed it was something like #9-24 UNC but those screws do not seem to exist (at least Wikipedia does not mention them and none of the retailers specialized in imperial screws have them). Do they exist in the US?

diameter length windings

  • The exact length of 30 mm gives credence to my hunch it is a metric screw. I have never heard of a 9/24 machine screw, bolt or pitch form. – Chris Taylor Feb 25 at 19:50
  • Not asked, but that type of screw is commonly called an "oval head machine screw". – JPhi1618 Feb 25 at 21:23
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A friend of mine pointed out that #9-24 UNC is actually a standard size – it is far from common these days (mind this is an old piece of furniture). Here is a table from 1916:
American Screw Co Table
The listed diameter of 0.176 inch amounts to 4.48 mm – well within my range of measurement error.

Now that this is cleared up, the real struggle will be to find one of these ancient screws, with sufficient length, more than 100 years later… in Germany.

On a side note: I did read the other answers and I appreciate them. I got my hands on a #10-24 UNC screw, but it does not fit.

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Machine screws below 1/4" are mostly found in even numbers.

Assuming a thread pitch of 24, it's likely a #8 or #10. Their diameters are listed as 4.166mm and 4.826mm, respectively.

Since M4.5 screws are only commonly made with .5mm and .75mm pitches, I'd lean toward the 10-24.

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There is no #9 screw size, it will be 8-24 or 10-24. 8-24 would be 4.06mm and not likely to fit your measurements because sometimes thread cutting results in slightly SMALLER diameters, but not larger. #10 would be 0.19" or .483mm diameter, so that fits your scenario better and 10-24 screws are much more common that 8-24.

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