Fix Broken Studs - an example
Not too long ago I had to fix broken studs on my rototiller. It's a pull behind unit that mounts on my garden tractor.
The vibration from the rototiller and the occasional jumping up and down on hard soil helped crack the top off of some hex bolts that were mounted inside of special head bolts.
The special head bolts help secure the head on the engine, and they are threaded on the inside to allow bolts to be fastened into them to hold on the fuel tank.
Photo below shows how the engine and tank assembly is supposed to look. The fuel tank brackets mount the tank on the head of the engine. This works well until you wind up with too much vibration that cracks the mounting studs.
With all the movement, the heads of the bolts that screw down inside the special head bolts snapped off, leaving my fuel tank hanging by the fuel line as I tilled the garden.
When I discovered the problem, I looked down at the studs stuck down inside the head bolts with no convenient way to get them out - they had snapped off flush with the top of the special head bolts.
Photo above right shows a head bolt on the right and the special brass colored head bolt that the studs screw down into on the left. The fuel tank mounts to two of the special head bolts. You can see the fuel tank mounting brackets on the far left of the photo.
Photo left shows how the hex bolts fasten down into the special head bolts on the top of the engine. The hex bolts hold to brackets in place. The top bracket holds the upper portion of the fuel tank in place. The lower bracket pushes the bottom of the fuel tank out so it doesn't wiggle on the top bracket.
So, it was time to fix these broken studs and get the rototiller back into action. Here is how I did it:
- collected up pieces from the "wreckage"
- removed the spark plug
- removed cowling from on top of the engine
- loosened and remove the special head bolts
- set up the drill press with a jig to hold the special head bolts in place
- used a drill bit that was just a little smaller than the broken studs I wanted to remove
- set the drill bit way up inside the chuck so it wouldn't flex when drilling
- used cutting oil and slowly drilled out the broken studs, making adjustments as necessary to the special head bolts to keep my drill bit centered
- removed the remainder of the broken studs with a stud remover and a round file
- cleaned out the threads on the inside of the special head bolts
- found hex nuts and lock washers that were the right thread size and depth to remount the fuel tank
- reassembled the head bolts and fuel tank
It all sounds easy and straight forward, but it can be a bit challenging. The two key steps that led to success were:
- Drilling exactly in the center and using a large bit. The larger bit was stiffer and it allowed me to stay centered on the broken studs as I drilled them out. This helped remove nearly all of the broken studs, leaving just a tip of threaded material at the bottom to deal with.
Photo right shows the drill bit deep in the chuck. This allowed me to drill out the broken studs with minimal flex of the bit.
To be successful, you need to drill very slowly with light pressure. Exerting higher pressure will cause the bit to drift, and then you risk drilling out the threads on the inside edges of the special head bolts.
Use of a drill press is required. You just can't hand hold a drill and keep it exactly centered on the work piece. The photo below shows a vice mounted on a drill press that allows the work piece to be centered and held still.
- Extracting the remaining threaded portion of the bolts using a stud extractor and a round file. The stud extractor worked well on the first threads, but bottomed out before getting a grip on the second one, so I found a round file that worked perfectly to grab the remaining threads and get them backed out of the threaded head bolt.
Note: once the studs were drilled through, it relieved quite a bit of pressure, and the remaining threads deep inside the head bolt were removed as if they were only finger tight.
Photo left shows the end of the broken stud that was extracted using the stud remover. The upper portion of the stud was consumed by the drill bit, and didn't need to be extracted.
Photo right shows how a round tail file was used to remove the broken stud. Any type of file can be a substitute for a stud remover.
An alternative fix for this problem would be to simply remove and replace the special head bolts, and then remount the fuel tank. I could have ordered the parts from the manufacturer, but that would have taken a week to get to me, and it would have cost about $20.
Instead, I stayed with my approach to frugal living and self reliance, and fixed it myself in about a hour, using scrap materials in my shop. The repair cost me nothing but my time. And, the whole operation gave me more experience with repairs like this, and thus provided me with more confidence to handle something similar the next time it comes along.
Done with Broken Studs, take me back to Do It Yourself