Broken Head Bolt Repair

[Don52]

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


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


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


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


5. Nut welded to head bolt

[Don52]

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


7. Cleats done


8. Cleats mounted to motor


9. Motor clamped to Bridgeport


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

[Don52]

11. Drilling out head bolt


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


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


-Don

[Electric4Life]

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!

[ryanjones2150]

Nice work, Don.