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Securing Trailer Bunk With Lag Screw Instead of Carriage Bolt

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  • Securing Trailer Bunk With Lag Screw Instead of Carriage Bolt

    I'm replacing a bunk on a trailer for a 9,000lb boat. The lumber is an odd size, 3"x4", that I couldn't source locally. I called the manufacturer and they had the bunk I needed, all carpeted, but without the carriage bolts installed. If I have to drill the bunk, countersink, and install the 7/16" carriage bolts, I will have to remove at least some of the carpet. While sitting on the trailer looking at the new bunk sitting on the brackets to which it will be fastened, it occurred to me that maybe lag screws driven in from the bottom could be used instead of bolts. That way I don't have to remove as much carpet and drill. I'm not an engineer. Would 7/16" lag screws be secure enough?

  • #2
    Every trailer I have owned has the bunks lag bolted from the bottom.
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    • #3
      And the 9000# boat will hold the bunk down when trailered.

      Just make a habit of checking the lag screws once in a while, as they can loosen up, especially when you pull the trailer without the boat on it.
      Medford, WI


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      • #4
        My 11K boat bunks are lag bolted from the bottom, and mine are 3/8. Being a 3x4 (2 each 2x4 glued together) there is plenty of wood there to work with
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        • #5
          mine are lagged in from the bottom with stainless lags. can't remember if they are 5/16 or 3.8 though

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          • #6
            Thanks, everyone. I will lag screw them. I mis-stated size of fastener. They're 3/8".

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            • #7
              If you really wanted to use carriage bolts you can use a razor blade to cut the carpet just where the bolts would go then glue the carpet back down. But I agree with the other posts that you'd be good with lag bolts.

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              • #8
                Lag bolts would be ok to use only if you countersunk the heads so the head is below the surface of the wood.
                Cutting the carpet and gluing it back down like suggested will be ok if you want 16 or more dimples in the bottom of your boat.

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                • #9
                  Originally posted by Idlespeedonly View Post
                  Lag bolts would be ok to use only if you countersunk the heads so the head is below the surface of the wood.
                  The lag screws fasten to the bottom of the bunk. The boat rests on top.

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                  • #10
                    I installed the bunk today using 3/8" x 2 1/2" stainless lag screws, fender washers and lock washers. I drilled 15/64" pilot holes before ratcheting in lag screws. The bunk seems quite secure, but the real test will be loading the boat on the trailer
                    Last edited by Four Winns 214; August 19th, 2017, 06:13 AM.

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                    • #11
                      the real test is a long tow and see if any shake out
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                      • #12
                        The 32 lag screws holding my 2 bunks on haven't fallen out in the 15 years I've had them

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                        • #13
                          Originally posted by Four Winns 214 View Post
                          The lag screws fasten to the bottom of the bunk. The boat rests on top.
                          Oops, I meant carriage bolts.

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                          • #14
                            I replaced my bunks this summer and used lag screws from beneath as this is how it was done prior. The first set of boards lasted about 10 years and hope this set of boards will last as long.

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                            • #15
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                              Stainless lag bolts have a smooth finish which would possibly make them a little more susceptible to backing out. However, galvanized lags have a very rough finish and will grip the wood. Also as wood dries it shrinks. I have actually broken galvanized lags backing them out because they hold so well. I don't know how it works but the wood sort of bonds to the galvanized. Use galvanized lags, after 6 months take one out and you will see what I mean, the shank will have wood stuck to it. Having said all that...since wood shrinks, you will want to re-tighten all the bolts after the first month or so and then you should be good til the bunks rot or you wear the carpet out.

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