Recently I've been working bringing a '94 4-R up to snuff. The undercarriage steel plate that guards the front of the vehicle was loose. I found that a bolt head had been snapped off while the bolt had siezed. I drilled it out and tried using a spiral extractor, but I snapped it off. Drilling with a cobalt tip, I can't make much progress on the hardened steel of the extractor. Is there any kind of drill bit that can penetrate it? I have also thought of finding a narrow cold chisel that can break up the extractor and allow me to remove it in pieces. Is that possible?

Recently I've been working bringing a '94 4-R up to snuff. The undercarriage steel plate that guards the front of the vehicle was loose. I found that a bolt head had been snapped off while the bolt had siezed. I drilled it out and tried using a spiral extractor, but I snapped it off. Drilling with a cobalt tip, I can't make much progress on the hardened steel of the extractor. Is there any kind of drill bit that can penetrate it? I have also thought of finding a narrow cold chisel that can break up the extractor and allow me to remove it in pieces. Is that possible?
A high speed carbide bit in a die grinder (very high speed) will grind it out but it takes a ton of practice to hold it dead steady and not let it vibrate and oscillate to the point where you shatter the carbide bit.

Good luck

Would it be possible to simply drill a new hole in the plate nearby the old broken bolt and drill/tap a new hole or nut in the matching frame surface to substitute for the missing bolt? I think it would be an easier approach and more likely to succeed.

JMHO

I had a similar issue with my bash plate. I drilled out the bolt and tack welded a nut to the frame and then used some washers to keep it tight and square on both sides. Does sit a few centemeters lower but it saves you taping it out.