How to Use Carriage Bolts in Metal

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Carriage bolts are made with a square seat beneath a round unslotted head. In wood, tightening the nut pulls the shoulders of the square seat down into the wood and prevents the bolt from turning as you tighten. In metal applications this doesn't happen unless the metal is very thin and a carriage bolt will rotate with no way to put a wrench on it. If you're stuck and don't have a regular bolt, here are a couple of ways you can use a carriage bolt to fasten metal objects.

Things You'll Need

  • Carriage bolt
  • Nut
  • Washer
  • Lock washer
  • Wrench
  • Metal adhesive
  • Drill and bits

Lock Washer

Find a lock washer large enough to slip over the shoulders of the square neck underneath the bolt head, then place it on.

Slide the bolt and lock washer through the hole and thread a washer and bolt to the other end.

Tighten with a wrench. The pressure from the lock washer as the nut tightens will hold the head of the bolt and prevent it from twisting in the hole.

Adhesives

Find a flat washer large enough to slip over the shoulders of the square neck beneath the head of the carriage bolt.

Apply epoxy, JB Weld or another metal adhesive to both sides of the washer and slip it onto the carriage bolt.

Insert the bolt and washer into the bolt hole.

Spread more adhesive underneath the bolt head if needed. Make sure there is plenty of adhesive between the bolt head and washer and between the washer and the metal surface underneath.

Thread a washer and nut to the other end of the bolt and hand-tighten. Allow the adhesive to dry and cure, then tighten the nut with a wrench. The adhesive will hold the nut in place long enough to get the bolt tight and the shoulders of the carriage bolt will dig into the metal and hold it as well.

Modifying the Bolt Hole

Measure the distance across the square neck of the carriage bolt diagonally from corner to corner.

Select a drill bit 1/8 to 1/16 of an inch smaller in diameter than the diagonal width of the carriage bolt neck.

Drill a hole wider than the bolt hole and 1/8 to 1/4 inches deep, depending on the depth of the square neck underneath the carriage bolt hole. The hole will be only slightly narrower than the square neck underneath the head of the carriage bolt.

Slip the carriage bolt through the bolt hole with the round head on the side where you drilled the recess hole. Slide a washer over the other end and thread a nut.

Tighten the nut slowly with a wrench. The pressure of the nut as it tightens will pull the bolt down into the recess hole, pushing the metal edges out of shape as the corners of the bolt neck are drawn downward. Unless the metal is very hard, the square neck will draw down into the recess and create a square hole for itself that will prevent the bolt from turning when you tighten the nut.

Tips & Warnings

  • Replace the carriage bolt with a standard bolt as soon as possible. These are only temporary jury-rigged solutions and should not be regarded as permanent.
  • Don't use carriage bolts to join parts that are critical to the safety of a machine or structure while in use, unless that structure is designed to accept carriage bolts.
  • Carriage bolts are designed for wood to wood or wood to metal joints. Don't trust them with your life in metal to metal joints because of the risk of loosening under movement, vibration or pressure.

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