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Thread: Broken Head Bolt Repair

  1. #1
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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
    Name:  1. Loose Connecting Rod.jpg
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    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. The broken stud is located on the upper right hand side in the following picture.

    2. Broken off head bolt
    Name:  2. Broken off head bolt.jpg
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    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
    Name:  3. Broken off head bolt close-up.jpg
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    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
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    5. Nut welded to head bolt
    Name:  5. Nut welded to head bolt.jpg
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  3. #2
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    Re: Broken Head Bolt Repair

    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
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    7. Cleats done
    Name:  7. Cleats done.jpg
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    8. Cleats mounted to motor
    Name:  8. Cleats mounted to motor.jpg
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    9. Motor clamped to Bridgeport
    Name:  9. Motor clamped to Bridgeport .jpg
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    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
    Name:  10. Head of Bridgeport rotated.jpg
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    Smith Oxyacetylene Torch
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  5. #3
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    Re: Broken Head Bolt Repair

    11. Drilling out head bolt
    Name:  11. Drilling out head bolt.jpg
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    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
    Name:  12. Cleaning up threads with tap.jpg
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    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
    Name:  13. Tapped hole cleaned up.jpg
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    -Don
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  7. #4
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    Re: Broken Head Bolt Repair

    Nice job. I've used that TIG trick a time or 2. The customer is always amazed

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    Re: Broken Head Bolt Repair

    Good job on the repair but for most it would cost more than a new engine.

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    Re: Broken Head Bolt Repair

    Very nice job documenting your project!

    Left hand drill bits are remarkably reliable when drilling out broken bolts or studs (if they are threaded).

    A big question is WHY did the rod cap bolts become loose? Rod bearing condition? Crankshaft damage?

    The oil looks pretty muddy.

    Thanks for sharing and pictures!
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  11. #7
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    Re: Broken Head Bolt Repair

    Really nicely done - I like the cleats too

    If you make yourself a big 90 degree fixture, I think maybe your cleats could have been used there and saved you the trouble of turning the head?
    Dave J.

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    Re: Broken Head Bolt Repair

    Nice job.

    Its amazing how sometimes a 10 minute job turns into 3 hours....
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    Re: Broken Head Bolt Repair

    Quote Originally Posted by crab fisherman View Post
    Left hand drill bits are remarkably reliable when drilling out broken bolts or studs (if they are threaded).
    I have purchased single left had drill bits for specific projects, but I never invested in a set. I didn't think that they would be effective in this case because the head bolt was over tightened and smashed into the imperfect threads at the bottom of the hole.

    Quote Originally Posted by crab fisherman View Post
    A big question is WHY did the rod cap bolts become loose? Rod bearing condition? Crankshaft damage?
    The oil looks pretty muddy.
    Yes, why did the rod cap bolts become loose? My friend John's theory is that the dirty oil caused excessive heat that could have caused the cap bolts to loosen up. Perhaps someone else on the forum has other better theories.


    Quote Originally Posted by MinnesotaDave View Post
    Really nicely done - I like the cleats too

    If you make yourself a big 90 degree fixture, I think maybe your cleats could have been used there and saved you the trouble of turning the head?
    Using a angle plate was my first thought, but as you can see the motor and cleats would require a large angle plate. I have some angle plates but they were way too small and I didn't have enough large scrap metal to cobble up one. It isn't a huge job to rotate the head and align it. I have the head alignment tool shown below that makes it pretty easy.

    14. Bridgeport head alignment tool
    Name:  14. Bridgeport head alignment tool.jpg
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    Quote Originally Posted by John T View Post
    Nice job.
    Its amazing how sometimes a 10 minute job turns into 3 hours....
    Yes, exactly

    -Don
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  16. #10
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    Re: Broken Head Bolt Repair

    Quote Originally Posted by crab fisherman View Post
    Rod bearing condition? Crankshaft damage? The oil looks pretty muddy.
    The cam shaft has plastic overmolded cams. The loose connecting rod bolts hit the plastic cams on the cam shaft and gouged it as shown below.

    15. Gouged cam shaft
    Name:  15. Gouged cam shaft.JPG
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    Smith Oxyacetylene Torch
    Miller Dynasty 200DX
    Lincoln SP-250 MIG Welder
    Clausing/Coldchester 15" Lathe
    16" DuAll Saw
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    Bridgeport
    Lincoln LE 31 MP & Lincoln 210 MP

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  18. #11
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    Re: Broken Head Bolt Repair

    My buddy was a mechanic at a motorcycle shop, but he grew-up in his dads welding welding business and was an accomplished weldor. He would remove broken bolts of all sorts in an aluminum engine by blasting them out with an O/A torch. For whatever reason, he never disturbed the aluminum threads.... (you had to clean them out, but they were There)

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    Re: Broken Head Bolt Repair

    Plastic cams...I have never seen that before...Chinesium?
    Do not believe everything that you think.

  20. #13
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    Re: Broken Head Bolt Repair

    Kawasaki engine? Some of the Deere’s that ran Kawasaki’s had plastic gears known to fail
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    Re: Broken Head Bolt Repair

    That was quite the process; thanks for bringing us along with you.

    I'd be concerned regarding the concentricity of the rod end; what with the loose bolts allowing it to pound back and forth on the crankshaft.
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    Re: Broken Head Bolt Repair

    Quote Originally Posted by farmshop View Post
    Kawasaki engine? Some of the Deere’s that ran Kawasaki’s had plastic gears known to fail
    I asked John and he said that it was a Briggs and and Stratton engine.

    -Don
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  23. #16
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    Re: Broken Head Bolt Repair

    plastic gear is in use here and there but never saw plastic cams!
    as mentioned, the big end and crank need to be checked for roundness. probably all out of whack me thinks.

  24. #17
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    Re: Broken Head Bolt Repair

    If you find yourself doing this kind of work frequently enough, keep your eyes peeled for a horizontal milling attachment for the Bridgeport. I have one for mine, and while I don't use it often, as I have a Kearney and Trecker 2K universal horizontal mill for big work, it's still a nice tool to have access to when you need it. Nice work! Fixturing and holding work is always the main battle.

  25. #18
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    Re: Broken Head Bolt Repair

    Quote Originally Posted by hawkfan9 View Post
    If you find yourself doing this kind of work frequently enough, keep your eyes peeled for a horizontal milling attachment for the Bridgeport. I have one for mine, and while I don't use it often, as I have a Kearney and Trecker 2K universal horizontal mill for big work, it's still a nice tool to have access to when you need it. Nice work! Fixturing and holding work is always the main battle.
    Thanks, I'll keep that in mind.

    -Don
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    Bridgeport
    Lincoln LE 31 MP & Lincoln 210 MP

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