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I went to a local building supply company - one that tends to have a better selection than the big-box places - seeking some Grade 5 carriage bolts.

They pointed me to bins of galvanized carriage bolts. I checked and did not see the 3 radial lines that normally mark Grade 5. They said "galvanized bolts do not have the marking, but rest assured they are Grade 5"; in fact, I believe they implied that galvanized are always Grade 5 (or higher).

I tend to think they are lying or misinformed, but thought I would seek clarification.

RustyShackleford
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  • I'm using these to hang an old-school Hunter ceiling fan, the kind that are cast iron and weigh about 50lb. With the poor quality of hardware I've seen recently, I think it's worth getting graded bolts. I can always order them from places like boltdepot, but I would like to source locally. – RustyShackleford Aug 21 '19 at 20:28
  • Do you need galvanized? Big box stores do sell grade 5 and grade 8 bolts. Auto parts stores are another source. – JPhi1618 Aug 21 '19 at 20:57
  • I don't need galvanized; that's just where they sent me when I said I needed Grade 5. – RustyShackleford Aug 21 '19 at 20:58
  • My assumption was that galvanized bolts are not graded and are low grade. Just go to an Ace or Lowes or Home Depot, but I'd be interested if anyone knows better about galvanized. – JPhi1618 Aug 21 '19 at 21:01
  • Those places MIGHT have grade 5 hex bolts, but not carriage. – RustyShackleford Aug 21 '19 at 21:18
  • If you're worried about the quality, you could test them before use. – Andrew Morton Aug 22 '19 at 14:34
  • How would you test them ? Just stress to failure, while somehow measuring the stress ? Don't think I have the equipment for that. – RustyShackleford Aug 22 '19 at 21:05
  • @RustyShackleford It's only 50lb being supported - try pulling them apart by hand as a test and make sure the nuts fit reasonably well. – Andrew Morton Aug 23 '19 at 10:53
  • I'm probably paranoid. If it wasn't moving. Imagine an unbalanced fan - that bolt is getting something pulling it from side to side over and over and over again, every second or so, for hours or days. Metal fatigue and all that. I don't have any scientific reason to think Grade 5 is good enough, and Grade 2 isn't, but I kinda consider Grade 2 to be Grade-nothing. – RustyShackleford Aug 24 '19 at 06:11

2 Answers2

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Highly unlikely that a Grade 5 bolt (90.000 psi yield, depending on size) would be galvanized. If electro-galvanizing were used there is a high risk of hydrogen cracking at that hardness level. If it were hot dip galvanized, the required temperature of about 800 F would temper the steel to a lower strength. This tempering affect can be fixed using alloy like chrome/moly, but now you have a pretty expensive bolt; not one available in a hardware store.

blacksmith37
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  • I believe you. Disappointed at this store for being misinformed and/or untruthful. As I said, it's not a big-box store. It's a locally-owned outfit catering to professional builders and is usually top-notch in knowledge and service (and in price :-) ) – RustyShackleford Aug 22 '19 at 21:12
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I have not seen a carriage bolt in grade 5 or higher. If you want a bolt that will resist rusting then you should use grade 8 bolts. They are so hard that rusting is almost non existent.

d.george
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    ??? What has hardness got to do with rust resistance. – Martin Bonner supports Monica Aug 22 '19 at 11:20
  • I'm not too worried about rust; somehow I've given that erroneous impression by mentioning galvanized bolts in my question. – RustyShackleford Aug 22 '19 at 21:08
  • At the risk of inappropriately mentioning a specific merchant, you can definitely get Grade 5 and 8 carriage bolts: https://www.boltdepot.com/Carriage_bolts.aspx – RustyShackleford Aug 22 '19 at 21:10
  • @Martin Bonner; if you ever get to disassemble a car or other piece of equipment that has been in a harsh environment, the "grade 8 bolts" that you find may be the only ones that are not corroded or rusted away. That is why I made that comment. I just surmised that the hardness was why they do not rust as quickly as other lower graded bolts. – d.george Aug 24 '19 at 10:22
  • @d.george It's almost 40 years since I studied metallurgy, but hardness and rust resistance are not related. – Martin Bonner supports Monica Aug 26 '19 at 08:23