Best method to drill out a broken off bolt

MattFL

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Oct 20, 2010
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Can anyone give me your favorite method for drilling out a broken off bolt? The part that makes me nervous is doing it free-hand, not under a drill press. Are there any tips or special tools for trying to keep the drill bit centered? Maybe a collar of some sort I can slip around the bit that keeps it centered in the hole? With the corrosion component I'm guessing an ez-out won't work, so the plan at the moment is just drill the bolt out and use a heli-coil.

I was trying to remove the exhaust cover on my 99' Honda 50 and sure enough the bolt broke right off. I was trying with a small 3/8 impact on the lowest setting, but either low wasn't low enough, or the bolt was weaker than I expected.


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dingbat

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In this case, I would use a transfer punch to center the hole then start with something like a 1/16” drill then bump up to 1/8” and so forth.

I drill until most the meat has been removed then try an easy out. If they doesn’t work, I drill with a tap drill and run a tap down to clear the threads
 

ahicks

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You can wrap electrical tape around the bit (or a center punch if so inclined) so it'll center in the hole. I start with a 1/8" bit - a nice sharp one. When drilling watch carefully for the bit to go through the bolt. There's an air space (3/16-1/4"?) you'll see/feel beyond the bolt. DO NOT DRILL ANY FURTHER! If you see alum. chips, stop drilling! Don't mean to make you too nervous, but you can wreck a block drilling too far...

Next time, use a propane torch to warm the area of the block where the stuck bolt lives. Don't worry about overheating the block. It's difficult to even get the paint to discolor. I cut the number of broken bolts in half when I started doing that....

Looking at the light side, you're doing an easy one. The one I dread is the lower rear. I have a trick for doing that one....

Do you have the head off? With that cover off, now is a good time to be cleaning all the water passages from this area to the head..... -Al
 

MattFL

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Thanks for the tips and warning, I'll try the tape around the bit. I was also looking at a small bearing or collar to insert into the hole that I could pass the drill bit through, to hold it in the center for the initial hole. I'll order some replacement bolts and measure the length, then mark the bit for that length. I'm hoping to drill it then use a helicoil or similar, without taking the plate off (don't have time now for 6 more broken bolts). Or I guess if the helicoil is too difficult due to the depth, I can just tap it for the next larger size.

The head is not off, I'm hoping to not have to remove it for fear of creating a bigger mess, given the age and salt use.

When I broke it, my goal at the moment wasn't to take the plate off, but to just test if I was lucky enough that the bolts might be free in anticipation of removing the plate in the near future. I was working on something unrelated and figured hey I have the impact in my hand, I'll just try it for a few seconds on low just to see if it moves, so I didn't bother with the torch just figuring if it doesn't come out easy then I'll get more serious later. I figured it might not come out, but didn't expect it to break so easily. Lesson learned!
 
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MattFL

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airshot

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As a 50 year tool maker, the best way is to get what is caller a center drill...made in a variety of diameters. You need one that is the same dia as the hole so it just fits in !! These are cheap and could probably borrow one from a local machine shop. The tip on these will create a true center small hole. Then follow up with a drill bit of the proper tap drill dia for the thread at hand. Then follow up with a tap of the correct size. Or use the drill and tap for a helicoil. They key here is to get a true on center starting hole...the center drill will do that even with a hand drill !! Sorry for the bold print, no idea why it changed !! Wish you were closer, I have everything you would need in my toolbox.
If the thread/ hole is in good shape, you could use a left hand drill bit after the center drill, the left hand drill will unthread the broken bolt as it bites into the steel. Let us now how you make out....
If your not familiar with a center drill, just look them up and you will see how they work....
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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I'm thinking about lightly tapping in one of these bearings to keep the bit strait during the pilot hole. I measured the hole in the plate and it's ~6.5mm, these bearings are 6.5mm OD, 3mm ID:


A solid collar may work better, but the only ones I found are in China with 2-3 week shipping, I can have the bearings in 2 days. :)
mcmaster carr has drill bushings available today.
 
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I'd go with the two options above as first and second choice. However, in a pinch I found a dowel of the correct diameter. I then drilled a hole in the center and used that to keep the bit centered.
 

MattFL

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Does anyone know the pitch of the thread on those bolts? I ordered new bolts but they're not here yet and I would like to order the helicoil too so it all arrives at the same time.

Update on the stuck bolt; I used a center drill to poke a hole right in the middle, the outside of the center drill just fit inside the hole in the plate and helped keep it centered. Then very carefully drilled a deeper hold with a I believe 1/8 left bit. I'll finish drilling it with the helicoil bit, and after the new bolts arrive so their length can be used to measure how deep I can safely drill. I'm too chicken to try a bolt extractor for fear that it will break off and I'll be in an even bigger mess, so it seems drilling the whole thing out might be the less riskier route.
 

MattFL

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Quick follow up - the new bolts arrived and they thread nicely into a 6x1 die, so for the next person doing this, the thread must be 6x1. :)
 

MattFL

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In case this helps the next person, I think we had success with the helicoil. I put a few drops of Rydlyme in the hole using a syringe, to remove the corrosion, that was a mistake I think. First it held and started fizzing like alkaseltzer, then suddenly the rydlyme disappeared and started coming out the side of the plate. Basically it ate the corrosion super fast. I flushed it with water and baking soda to stop the reaction, and the metal looked new inside, corrosion all gone.

Next I used the center drill to put a hole right in the center of the bolt. The wide part of the bit was almost exactly the diameter of the big hole making it easy to stay centered. Then I used a left bit to put a hole right down the center of the bolt.

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I tried an EZ-out on the bolt briefly but it wasn't moving. So as not to risk a broken ez out, I drilled it out to 1/4 inch and ran in a helicoil. The wanted to grab hard and so I went really slow and patient so the bit didn't break:

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Success! So far it appears to be holding and no leak as of yet, taking it off shore tomorrow, wish us luck! I'm not going to try removing the other bolts unless/until there is a very specific reason.

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MattFL

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Good Luck!! Nice pics of the work!

Thanks, the tips here for the center drill, to be patient and the limited depth of the hole were golden. All 3 tips were instrumental in success. The motor is wet in the pics because we were working in the rain. We took it off shore today and the motor ran great (also put in a new timing belt and tensioner while we were in there, and all the valves were still in spec after several hundred hours since the last check). Unfortunately the weather was not great and my wife was not happy, declaring never to go in the ocean in this boat again LOL. The storms were rolling in and with the cold breeze came the predictable pick up in the seas so we got drenched on the run back to the inlet, but we beat the rain!
 
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