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I recently drilled and tapped a 5/16"-18 in a piece of soft steel, then found that the 5/16"-18 bolt I had chosen wouldn't fit (the bolt was slightly too large). When I found that other 5/16"-18 bolts fit just fine, I got curious and eventually tried every 5/16"-18 bolt that I had. I was amazed to find thirteen different sets of letters (and sometimes numbers).

The one bolt marked "AKD" was slightly too big to fit. Of the four marked "HKT", two fit and two did not (too big). All the others fit just fine (though with slightly differing amounts of slop); they were marked "AFE", "AHB", "AKD", "AMD", "BL", "FH", "HBJ", "HBN", "HBS", "HHW", "HKT", "307A CYI", and "307A JZ". A few had no letters at all. I got these bolts at different times and places over an unknown number of years, at various hardware stores and big home centers. They are all steel, not galvanized, not stainless (as far as I know), and all the threads are the same (18). None of the heads bear any of the lines that are supposed to identify the grade.

I've been searching the internet, and I found many sites that claim to explain these codes, but so far none of them include the codes I found on my bolts. Can anyone point me to a comprehensive reference for these codes?

  • The short answer is that they are proprietary inventory codes. – isherwood Apr 14 '18 at 19:31
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Those sets of three letters sound like a whole slue of manufactures as seen here (fastenal.com) in the example of a head marking, on page 44. The Appendix, beginning at page 36, shows industry standards. Any bolt not conforming to these specifications is assumed to be a grade 2 or less.

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The three-letter code is the manufacturer code (for example, HKT stands for Hau Kwang Enterprises) and the numbers (if metric) or radial lines (if SAE) refer to the bolts' strength. If there are no numbers or lines present, assume the bolt is no better than a grade 2 (low strength).

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Are you certain that the ones that did not fit are actually 5/16-18 UNC and not M8 x 1.25? The 8 millimeter threads will often seem to start in a 5/16 tapped hole but will quickly interfere. The outside diameters are only 0.002" apart and the thread pitches are only 0.006" different. You would be hard pressed to detect that by eye.

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The letters are from the manufacturer mostly in combination with the material grade.

Why the bolt did not fit ( in my guess ) is that you have different types of thread .

UNC / UNF / UN

Coarse / Fine .. even though the size is the same the TPI ( threads per inch ) can be different.

  • Hello, and welcome to Home Improvement. Thanks for the answer, but it's a little hard to understand; you might want to edit it to make it clearer. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jun 26 at 1:13
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I know this is an old question thread, but the letters (and sometimes symbols) on a bolt refer to its' manufacturer. Here I've included a link to the best resource you'll ever find or ever need on the subject - The DEPARTMENT OF DEFENSE HANDBOOK LISTING OF FASTENER MANUFACTURER’S IDENTIFICATION SYMBOLS.

This .pdf file from the US Department of Defense lists every bolt manufacturer that ever existed, quite literally. If you can't find it here, it was probably made by aliens! An example is "HHW", which means it was manufactured by Ho Hong Works in Taiwan (page 152 in the .pdf document - page 105 as noted at the bottom of the document itself). Here's the link:

https://quicksearch.dla.mil/Transient/B11EB30153DE40B48DA8DA80AD186E9D.pdf

(PS: If you happen to get a "This site is not secure" message, it's nothing to be concerned about. Just click "details" and then "Go on to the webpage". This is a military - .mil - website that simply has a website certificate error. I assure you that the site is perfectly safe - I've been on it many, many times.)

  • Does that include those made in the UK, China, India, Germany etc? – Solar Mike Aug 1 at 9:33
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    I haven't searched the entire document, Mike, but I'm sure it includes manufacturers from around the world. The .pdf file is from the DOD Defense Standardization Program that has been cleared for public release. – chuckiii3 Aug 1 at 10:12
  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Aug 1 at 10:21

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