Drilling through a grade 8 bolt

[Xelaw]

I sheared a couple of bolts off when removing my head. There are hard grade 8 bolts and I'm not having much luck drilling through them. I'm using cobalt bits, I'm drilling slowly and I'm using machine oil to keep it cool. Does anybody have any tips for me?

[BuzzStPoint]

When you drill a Gr 8 bolt, you start out slow to get your groove.
Then you have to use high speed.

Plus use cutting oil. (motor, 3in1 or 2 cycle oil is not cutting oil)
Have someone keep putting the oil on the bolt/drill bit as you drill.

[CharlieB]

Make SURE that you center punch DEAD CENTER then use a 1/8 bit to drill a 'pilot' hole, very careful to maintain that center.

LOTS of oil, moderate speed.

Once the pilot hole is thru then advance to a 1/4 bit, again, MODERATE speed and lots of oil.

Too much speed, even with oil, will dull the bit and work 'harden' the bolt even further.

Often I use my left hand twist drill bits, on occasion when drilling the second (or third) step the bit may 'snag' the bolt and back it out.

Don't count on always getting that lucky.

Go slow and with LOTS of caution, that aluminum cuts like butter if you veer off to the side!

Drop the lower cowling if it at all gets in your way.

[Xelaw]

My problem was that my drill bits were crap. I was using an OEM brand screw extractor set that came with ez outs and cobalt bits. I'm sure the bits would have worked fine with normal steel, but not against grade 8 bolts. I went to Ace Hardware and bought another 1/8" cobalt bit just to see and it cut through it much easier. I haven't gotten all the way through the first bit, but I'm getting there. Need to buy a few more bits tomorrow.

Also, I bought a diamond tip Dremel bit. It isn't much good at drilling through the bolt, but it's very effective at widening the hole. I was a bit off center and was able to correct it easily with my Dremel using this bit.

[pnwboat]

Carbide drill bit will have no problem drilling through a grade 8 bolt.

[Shife]

Carbide drills, low speed, don't skimp on the cutting oil. As mentioned, use real cutting oil. 3 in 1, Quaker State, and WD40 ain't what you're looking for.

[Xelaw]

Cobalt bit plus WD-40 worked quite well. Now the question is, how do I get the remaining part of the bolt out of the threads?

[hismajst]

Use "Aquacut Natural" as a cutting oil. I have access to all the best cutting oils and this would be the first one I grab every time. Best stuff on earth! If everything starts going south get your hands on a good carbide deburring bit to hog er out. How deep of a recess are we talking about before you hit the end of the broken stud?? Snap On makes some REALLY tuff extractors, but don't skimp on the extractor as removing a broken extractor can really make life miserable!!!

[RRitt]

My problem was that my drill bits were crap. I was using an OEM brand screw extractor set that came with ez outs and cobalt bits. I'm sure the bits would have worked fine with normal steel, but not against grade 8 bolts. I went to Ace Hardware and bought another 1/8" cobalt bit just to see and it cut through it much easier. I haven't gotten all the way through the first bit, but I'm getting there. Need to buy a few more bits tomorrow.

Also, I bought a diamond tip Dremel bit. It isn't much good at drilling through the bolt, but it's very effective at widening the hole. I was a bit off center and was able to correct it easily with my Dremel using this bit.

there is a thriving cottage industry that resharpens aviation bits and sells them on Ebay by the hundred. Apparently they use a lot of rivets building and repairing aircraft and they love cobalt. I buy 100 cobalt bits for under $20. The size varies since they are for pilot holes but it is always around 1/8" x 4"long. A drill size chart with metric and letter sizes helps. At the very first hint of dull I throw the bit away and grab another. The bits are so cheap and the time saved so big that I don't mind throwing away 4-5 bits for just one hole.

[tjello327]

All that I ever do to get out broken bolts like the one you have is to drill the hole out to the proper tap drill size, then tap out the hole with the correct tap. This is if an e-z out don't work first, and the pilot hole is on center. I also use a good center drill first. It will drill about 3/8" deep into the bolt with no problem. As for using carbide, the faster you run it, the better. Too slow and carbide will break much easier.

[foodfisher]

Cobalt bit plus WD-40 worked quite well. Now the question is, how do I get the remaining part of the bolt out of the threads?

Don't laugh, but, when I got down to where I could see the high part of the female thread with a dremel grinder, I stopped and peeled the remaining metal out of the threads with a pick. Retapped afterward. PIA I know, but it worked.

[Xelaw]

Well, it's come to this: http://forums.iboats.com/force-chrysler-outboards/machine-shop-question-407512.html#post2726716

[ram345]

dude,if youve drilled the bolt most of the way either retap or install a helicoil ,making sure youre new bolt doesnt bottom out on whats left of old bolt ,rule of thumb is 1 and half the depth of the size of the bolt!