For a broken 3/8 stud use a 5/16 nut. Aim the mig wire for the center of the stud (need to make sure you have adequate heat) then use a circular motion to finish the weld. Works well. The broken stud can be deeper than you think.

One time in work I saved a bunch of money extracting a large broken bolt in a very large pump base. It was several inches in the hole. I took a new bolt and drilled the center out. A good welder used a stick machine to weld the new bolt to the broken one. Used bees wax once the welding was done and it came out like butter.

I understand that a simple masonry bit has a carbide tip on it in an emergency.

After welding on the nut,
use cold to help remove the broken stud:

https://www.autozone.com/greases-and-gea...rant/898352_0_0

Holding dry ice against the nut before turning might also work.

I would drill a smaller pilot hole with a regular drill bit. Deep enough for the next size to follow. Then go at it with as large of a reverse bit as you feel safe with. It doesn't hurt to have soaked the stud with penetrating oil or such before hand to help it come out.

I had a recollection just now, about welding. I'm using thin .025" wire, and I think when I've tried to crank the amps up in the past, the thin wire just burns out without welding. I'm going to try a few attempts with .030" or .035" at higher amps and see if I can get those welds to stick to the stud.

Just in case I picked up left hand bits, transfer punches, a hinge-drilling kit (keeps drill centered in hole) in case welding with the heavier wire fails.

The more you weld in there the greater the chance you screw up the threads and you'll never get the stud out. If it was broken flush to the head surface I'd say weld with a vengeance but when its down in the hole I don't think welding is a good idea.

All the advice from everyone is good except they seem to forget one thing.....Its a 1973 360. im not even gonna remind you of how old that is. your best bet is to drill out the hole and heli coil it to stock size. that is 5/16. If you cant drill it strait pay someone who can or pull the head and have an engine shop do it. Ive removed hundreds of broken ext. studs and bolts out of all kinds of heads and never had good luck doing That stud they are very difficult to say the least. metal stud iron head. you would think it would come right out but that is not the case with small block mopar outter ext studs. good luck

I usually don't try welding on a nut unless the broken bolt or stud is sticking out of the hole. But having removed lots of broken bolts this is how I do it. First their are many methods some work when other won't, so you have to play your odds. If you break off a bolt when tightening, then easy outs and left hand drill bit work good, as long as the tension on the treads is removed when the bolt breaks. If the bolt is rusty or cross threaded you need to drill it out. First center punch, exactly in the middle, then take a small drill bit like a 1/8 and drill always through the bolt. Then take the right size to tap the bolt and drill it, the big bit will follow the small hole and it won't wander off center. When drilling in the car, I screw another bolt in a close hole so I have a point of reference to keep the drill straight and square. If you have drilled it in the center a tap can then be used to pull out the remaining threads of the bolt. If its off a little or the threads are junk drill to heli coil size and repair is done. Heating a bolt usually makes it harder, so I have to decide if heating or welding a nut is the best chance, because if I then have to drill it makes the process more difficult. And of course use cutting oil when drilling to save your bits and help keep them from wandering.

I use 0.025" for this type of work. I also have a 0.035" larger welder but the smaller wire is the ticket.

Practice first. Remember that you get more burn by adding wire speed not voltage. But both volts and feed have to be set correctly.

You don't have to be scared of it being in the hole. The 1 size smaller nut and aiming for the center with a good burn and then circular motion works very well.

Thanks Mark for the welding tip. I'm definitely amateur with welding to bolts. I already switched my machine over to .030" so I'm going to try with that for now.

I use the welding method all the time. just did a 93 360 in the truck. had 4 broken. If they are down in the hole I use a washer first then nut. It takes a few times sometimes to get them out. But the repeated heat is the key.

Tried welding a couple more times, and still couldn't get the weld to stick to the old stud when turning the nut.

Anyway I center punched it, then worked my way up through a few sizes of center drills, then drilled all the way with a thin drill. Real easy. Then used some left handed drills to get most of it out, then a regular drill close to the actual size. Finally ran a 5/16 x 18 tap through it with some oil, and that took the remaining bolt fragments out of the thread. Threads are good shape and it's back together.

Been down that road a few times at least. Good thing you have it covered.

I usually start with drilling out the broken bolt, saves a lot of time messing around before hand.
Happy to see you got it done. Gene

i tig weld metal to stud then use a nut welded to the metal. never had a weld brake. stud comes out without any problems.

I have blown them out with a torch, chase the threads and install a new bolt/stud