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What causes lag bolts to break?

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  • What causes lag bolts to break?

    I have used galvanized and non-galvanized lag bolts and a good % of the time the head will snap off or the shaft will shear. I even drill pilot holes beforehand. Do they make stainless steel lag bolts and are they any sturdier when tightening?

  • #2
    Stainless are better than Galvanized. You may need a little bit larger pilot hole and maybe a little less torque on your driver.
    info for all: --- "I like long walks, especially when they are taken by people who annoy me."


    • #3
      Grease 'em!

      When I have a bunch of galvanized lags to install, I usually get out the can of Johnson's wax that I use for my cast-iron tool surfaces. I just stick each screw into the wax enough to pick up a bit on the first few threads.

      The bottom line is that if you are snapping off more than the occasional lag screw, you are either setting your driver torque too high or the hole was not drilled properly. Or both. If you have to set the torque high enough to snap some bolts, the holes are not properly drilled.

      You need to drill a clearance hole through the piece you are attaching (Like a 3/8" hole for a 3/8" lag screw, and the correct thread-size pilot hole into the piece you're attaching too. The pilot hole should go as deep as the length of the lag screw.

      BTW, stainless, contrary to popular opinion, is a soft metal compared to steel. SS lags might work better without snapping just because they are smooth as opposed to the rough surface of HD galvanized.
      Unanswered Questions
      are far less dangerous
      than Unquestioned Answers.


      • #4
        We were hand tightening them w/ a socket wrench & using the diam drill bit the the plasma TV mounting bracket mfr called for, but maybe the wax might help or even going up a size on the pilot hole bit. We even thought about using a little shorter lag bolt b/c it would invariably snap towards the end of the tightening cycle.


        • #5
          A few years ago, I ran into this problem with buying hardware for the deck I was building at the time. With only two stores in the area that sold hardware of this nature, only one had bolts that were of good quality.

          While I planned everything out and purchased all the bolts I thought I needed at one time, I found myself short. I went to the nearest hardware store and bought the rest, and two out of the three snapped. The third I took to work and had the met lab test. Although identical in appearance, it had neither the proper tensile or shear strength that it should have had. I complained to the store and they didn't seem to be aware. This was before we had a Home Depot or Lowe's in the area, so I don't know how their hardware is, but I would suspect that they have taken steps to ensure some level of quality. I know that certain "high-strength" bolts have markings on the head, but I don't remember what they were, or even if that applies to "deck hardware".

          While I can't say that this is your particular problem, it is something to be aware of. Two bolts, viewed side-by-side, may look the same; but, the metal may be different, as can be the tempering process. I know that the problem was such, that the company that I used to work for had to increase its "sampling" on vendor supplied hardware, thus adding another technician to the metalurgical lab. It also made the decision to manufacture all of it's larger threaded hardware in order to ensure the necessary safety of the equipment.

          Last edited by CWSmith; 05-07-2006, 11:04 AM.


          • #6
            Lag bolts snapping

            So right you are. We have to be very aware of the QUALITY of both the hardware and the tools nowdays. I'm seeing more and more hardware coming through that is made in CHINA. China stuff = POOR QUALITY. I have even seen SANDPAPER that was made in CHINA and it didn't last more than 10 minutes in a little palm sander. As for grade stampings on hardware, look at the very top of the hex head. No marks at all = the lowest grade, Hash marks - spiraling around the top of the hex indicates stronger tensile strength. US bolts use a grading system with six stripes indicating Grade 8; three stripes indicate Grade 5; two perpendicular stripes indicate stainless steel. No stripes usually indicates Grade 2, which is what is usually found in a typical hardware store and should be enough strength for the clamping forces needed in general carpentry work.
            Jim Don


            • #7

              I took those markings for granted, but now I know what they mean. All the screws @ Lowe's are Hillmans, mfrd in Taiwan.


              • #8
                "Did you put the yellow key in the switch?" TOD 01/09/06


                • #9
                  Are lag bolts available in these grades, as I have not seen them w/ the markings @ Lowe's, other than S/S?


                  • #10
                    I'm not positive at Lowe's but i'm sure it's similar to Home Deopt.

                    At home depot all the standard (hex, carrage lag) bolts are Grade 2.

                    The only exceptions are the fine thread hex bolts are Grade 5 and should be labeled as such and most Home Depots should also carry a very small selection of Grade 8 bolts as well


                    • #11
                      I don't think they mfr a lag bolt > a Grade 2, unless you count S/S.


                      • #12
                        I have NEVER had a lag bolt snap and I almost never predrill. I just use my ridgid 1/2" impact. And most of the time I use stainless steel lags. You can get them from
                        Colorado Deck and Framing - When perfection is demanded