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Drilling out broken bolts

T

tomf

Regular Contributor
"Hi all,

Among other proje

"Hi all,

Among other projects, I'm currently engaged in the task of drilling out a bunch of snapped-off cover bolts from an ill-kept 15hp powerhead. I presume that if they come out reasonably clean, I can just rethread for the next larger bolt size, and if not, I'll be looking at helicoil-type thread inserts to get the thing back together.

However, this still leaves the question of how to get the corroded little suckers out in the first place. Any technical tips on how to best accomplish this? I'm having some trouble keeping the drill bit on target as it tends to veer off into the softer stuff around the bolt. Dremeling with a dentist-style bit seems to work a bit better, at least as long as the work is still near the surface.

Any special pointers from all Ye Olde Engine Gurus?"
 
"If you have enough left to gr

"If you have enough left to grab with a vice grip or a stud extracter, you can try plenty of heat. If not you need to file or grind the end of the fastening so that it is flat. Then you hit the center with a pointed punch. That should keep the drill bit steady. Start with a 1/8" cobalt bit, followed by a 3/16" bit and so on."
 
"OK, thanks. I'll have to

"OK, thanks. I'll have to try starting with a smaller bit and working up from there. The bolt heads are all snapped below the surface level so grabbing them is unfortunately not an option.

Do you usually use any kind of lubrication for this task?"
 
"Hi Tom, Tim here. I use a lef

"Hi Tom, Tim here. I use a left handed drill bit once the center hole is drilled. Use some penetrating or cutting oil as you work. Sounds like things are a bit messed up already. The most critical task is to get that first hole directly centered and 1/8" gives you a strong but small size to start. A premium quality bit is a must......once you get off center then your problems begin. Good luck!"
 
"Yeah, messed up is right. <[i

"Yeah, messed up is right.
happy.gif
Not even sure if this thing is worth putting back together, but at least if I can get it disassembled first I'll be able to evaluate my chances. Thanks for the input guys."
 
"If there's a stub of the

"If there's a stub of the bolt left, you can center your drill with the following trick:

1. Find a short section of pipe that you can drill out so that it's the ID of the broken off bolt. You want a tight fit.

2. Position the pipe on the bolt, then drill the bolt enough to make a spot on it.

3. Remove the pipe and drill the bolt all the way through with an 1/8 " bit, then use larger and larger drills until you reach the tap drill size for the bolt. (Going up in small steps keeps the hole centered in the broken bolt.)

4. Finally, tap out the hole.

Jeff"
 
"Jeff and Timothy have both be

"Jeff and Timothy have both beaten me to what I was going to say - the pipe trick and the left hand drill bit. I've found that a lot of the time the left hand bit will actually grip into the broken stud and wind it out while you're drilling.
I'd try using the original size tap first, before going oversize. Works more often than not"
 
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