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Broken Head Bolt Repair

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  • Broken Head Bolt Repair

    My friend John's buddy brought his lawn mower to a lawn and garden repair center because it was making a clattering noise. The center told my friends buddy that there was a broken connecting rod and the mower wasn't worth fixing. When John took apart the engine he found that the original problem was that the connecting rod bolts were loose, which would have been an easy problem to fix.

    1. Loose Connecting Rod
    Click image for larger version  Name:	1. Loose Connecting Rod.jpg Views:	3 Size:	66.5 KB ID:	610389

    During the reassembly of the head, the center broke off one of the head bolts. In fairness to the center the bosses on the head are different heights and some of the holes are through and some were blind. To make up for the different heights, spacers were added to the shorter bosses on the blind tapped holes, to keep the head bolts from bottoming out on the imperfect threads on the bottom of the hole. I believe that the center didn't put a spacer under the head of the head bolt on a short boss with a blind hole and kept tightening the head bolt after it bottomed out and broke it. It is the upper right hand hole in the following picture.

    2. Broken off head bolt
    Click image for larger version  Name:	2. Broken off head bolt.jpg Views:	3 Size:	101.2 KB ID:	610390

    Here is a close up of the broken head bolt. you can see that the break is about 1/8" below the surface and you can also see that all of the holes are counterbored.

    3. Broken off head bolt close-up
    Click image for larger version  Name:	3. Broken off head bolt close-up.jpg Views:	3 Size:	61.8 KB ID:	610391

    My go to method for broken off bolts is to TIG weld an extension to the broken bolt as shown below and then to weld a nut on the top of it. This method works very well for rusted bolts because the heat from welding expands the bolts and crushes the rust on the threads, to create some clearance.

    4. Head bolt built up
    Click image for larger version  Name:	4. Head bolt built up.jpg Views:	3 Size:	57.3 KB ID:	610392

    5. Nut welded to head bolt
    Click image for larger version  Name:	5. Nut welded to head bolt.jpg Views:	3 Size:	60.8 KB ID:	610393
    Last edited by Don52; 08-30-2020, 09:38 AM.
    Miller Thunderbolt
    Smith Oxyacetylene Torch
    Miller Dynasty 200DX
    Lincoln SP-250 MIG Welder
    Lincoln LE 31 MP
    Lincoln 210 MP
    Clausing/Colchester 15" Lathe
    16" DuAll Saw
    15" Drill Press
    7" x 9" Swivel Head Horizontal Band Saw
    20 Ton Arbor Press
    Bridgeport

  • #2
    Unfortunately my welded nut wasn't strong enough to extract the broken off portion of the head bolt. My welded nut broke off which meant that I would have to drill out the broken off head bolt. To support the motor I made some cleats to help me to clamp the motor to my Bridgeport.

    6. Counter boring cleat
    Click image for larger version  Name:	6. Counter boring cleat.jpg Views:	0 Size:	81.0 KB ID:	610395

    7. Cleats done
    Click image for larger version  Name:	7. Cleats done.jpg Views:	0 Size:	129.5 KB ID:	610396

    8. Cleats mounted to motor
    Click image for larger version  Name:	8. Cleats mounted to motor.jpg Views:	0 Size:	163.4 KB ID:	610397

    9. Motor clamped to Bridgeport
    Click image for larger version  Name:	9. Motor clamped to Bridgeport .jpg Views:	0 Size:	92.8 KB ID:	610398

    The flat surface on the motor was 90 degrees to the head so I had to rotate the head of the
    Bridgeport to drill out the broken off head bolt. I used a 1/4” drill to remove the center portion of the broken off 5/16”-18 UNC head bolt, without damaging the tapped hole in the head.


    10. Head of Bridgeport rotated
    Click image for larger version  Name:	10. Head of Bridgeport rotated.jpg Views:	0 Size:	166.2 KB ID:	610399
    Last edited by Don52; 08-30-2020, 09:40 AM.
    Miller Thunderbolt
    Smith Oxyacetylene Torch
    Miller Dynasty 200DX
    Lincoln SP-250 MIG Welder
    Lincoln LE 31 MP
    Lincoln 210 MP
    Clausing/Colchester 15" Lathe
    16" DuAll Saw
    15" Drill Press
    7" x 9" Swivel Head Horizontal Band Saw
    20 Ton Arbor Press
    Bridgeport

    Comment


    • #3
      11. Drilling out head bolt
      Click image for larger version

Name:	11. Drilling out head bolt.jpg
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ID:	610401

      Next I wound out the portion of the screw that was still in the female threads. (The portion that I removed looked like a Helicoil). To clean up the threads, I ran a tap through the hole..

      12. Cleaning up threads with tap
      Click image for larger version

Name:	12. Cleaning up threads with tap.jpg
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ID:	610402

      The Bridgeport allowed me to have the rigid setup and center the drill bit on the broken head bolt. As a result I was able to drill that bolt without damaging the original threads in the Aluminum head, for the first time. If the drilled hole wanders, which it often does, I have to drill oversize and use a Helicoil.

      13. Tapped hole cleaned up
      Click image for larger version

Name:	13. Tapped hole cleaned up.jpg
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ID:	610403

      -Don
      Miller Thunderbolt
      Smith Oxyacetylene Torch
      Miller Dynasty 200DX
      Lincoln SP-250 MIG Welder
      Lincoln LE 31 MP
      Lincoln 210 MP
      Clausing/Colchester 15" Lathe
      16" DuAll Saw
      15" Drill Press
      7" x 9" Swivel Head Horizontal Band Saw
      20 Ton Arbor Press
      Bridgeport

      Comment


      • #4
        I think you need a YouTube channel. This is great stuff. Thank you for sharing!
        ....I have always left the nut off and used a pair of locking pliers to back the broken bolt/tap/stud out.
        I'm digging the set up on the horizontal. well done!

        Comment


        • #5
          Nice work, Don.

          Comment

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