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OT: Carriage bolt installation help.

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  • OT: Carriage bolt installation help.

    HI Group,

    I have the rear bumper off my 1967 car because it needed the bolts replaced. I looked at chrome and at the price they wanted I had to look for the second choice. So, I got regular carriage bolts that have a SS cap that has been mechanically folded over on the edge for a very fine fit and look.

    Ok, to the question. The hole in the bumper is not the same size as the bolts I got for the square part of the carriage bolt. The originals had 2 sided flats on a round bolt. So how do you hold on to a carriage bolt to tighten when it spins in the hole in the bumper it's self? They are both 7/16" bolts so the hole is mostly full but the new bolts do not grab any of the sides to get a grip to tighten the bolts.
    I don't want to grab the head with the SS cap as that would damage the edge and be an issue. When you tighten the bolt enough to butt up to the bumper bar there isn't enough of the square to grab on too to finish tightening it up.

    I'm open to any good ideas other than bye them in chrome, because that is just not in the budget at over $6.00 each.

    Mr fixit for the family
    Last edited by Mr Fixit; 06-10-2022, 05:14 PM. Reason: age of car

  • #2
    Use an impact wrench.


    • #3
      I do not know any magic tricks for holding those carriage bolts. Perhaps you could fabricate some type of washer-like device which has wings to take up the slack. It would be best to install that device on the BACK of the fender so it does not leave the head of the bolts raised above the normal fender level. Think of a washer with two sides folded at 90 degrees.

      BUT, I don't know where you were shopping for them, but McMaster has 7/16" carriage bolts in stainless for more reasonable prices. You did not mention the length, but almost every length costs less than $5. Yea, I know you will also pay shipping.

      I am sure a hardware store could come close to those prices if you buy several. And have you looked at the local junk yards?
      Paul A.
      SE Texas

      And if you look REAL close at an analog signal,
      You will find that it has discrete steps.


      • #4
        Tack a bit of weld on the sides and grind down to be the flats that you need.


        • #5
          Cover the bolt threads in grease. Buy a couple of tubes of J.B.Weld and put lots of it under the head of the bolt. Shove the bolts into place and wipe off any excess J.B. Weld. Wait 24 hours. Run the nuts on over the greased threads and tighten them up with a wrench.
          Brian Rupnow
          Design engineer
          Barrie, Ontario, Canada


          • #6
            Magnetise them


            • #7
              A picture would be helpful. Have you tried 1/2" bolts?
              Paul , P S Technology, Inc. and MrTibbs
              USA Maryland 21030


              • #8
                If you know you won't need to remove then I rather like the idea of the JB Weld epoxy. But if you want to be able to remove them then I think I'd spot weld on some lumps that can be filed or carefully ground to fit the openings.

                And now you know why they are $6 each too. They're special hardware.
                Chilliwack BC, Canada


                • #9

                  Buy them and inch longer and assemble them hand tight. Lock 2 nuts on the outer end and use them for the hold back wrench.


                  • #10
                    Brian has it- though instead of JB, I'd just use 5 min. epoxy. Assemble finger tight, then at some point over the next 15 min or so, wrench on the nut to bring the parts up to contact while the epoxy can still be molded. After it's hard, you can tighten the nut up all the way. The epoxy in this case is now acting as a filler to keep dirt and moisture out, and plays no part in holding the join together. The nut and bolt hold it together like they would have anyway- and this will come apart if you needed to do that.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                    • #11
                      Bolt Depot has 7/16"-14 chrome plated steel carriage bolts up to 3" long, cheaper than 18-8 SS:

                      Click image for larger version

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                      Details (for the 3" ones) so you can check the square dimensions against your holes:


                      Click image for larger version

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                      4 (so you have a couple of spares, or to do the front bumper, too), 3" 7/16"-14 Chrome to Portland would be $31.44 + $4.95 (USPS) or $12.96 (UPS ground).


                      Avid Amateur Home Shop Machinist, Electronics Enthusiast, Chef, Indoorsman. Self-Proclaimed (Dabbler? Dilettante?) Renaissance (old) Man.


                      • #12
                        It seems as if the problem is, that standard carriage bolts don't work, because the holes in the bumper are made for a special bolt, and not for the square section of a standard carriage bolt.

                        I don't think buying standard carriage bolts in stainless, or in chrome plated finish, will make any difference.

                        I think he needs to get the special bolts that fit his bumper, or modify standard carriage bolts to work.


                        • #13
                          Here's a video that shows how a machinist would solve the problem.


                          • #14
                            Screwdriver slot in the threaded end if you can get at it when in place. If there is not enough free thread for the locknut idea from Deltap above you can put nut on the short exposed end to say 1.5 turns the insert another bolt in the back of that to lock it ie the bots end up end to end each half into the one nut.


                            • #15
                              Originally posted by deltap View Post
                              Buy them and inch longer and assemble them hand tight. Lock 2 nuts on the outer end and use them for the hold back wrench.
                              I was going to suggest grinding two flats on the end of the threads and holding with vise grips, but this is better plan.

                              leave the double nuts on the bolts as a clue for the next person that wants to take the bumper off.
                              When I get Time... I'll...