When a member of this list sends an e-mail to email@example.com, it is then relayed to over 300 other participants in the group. Each month a lady from England and a barefooted Texan pick a few of the more pertinent and interesting stories to pass on to the readership of GEM. This month, the original question came from Australia, the advice came from around the world, then we also had the verdict on several of the methods suggested.
First, the original question:
I have a bit of a problem with steel studs. The thread is frozen with rust in a cast iron exhaust manifold, and despite the application of the oxy torch and penetrating oil they still refuse to come out. The studs are rusted down to about half their original diameter, so I cannot give them a good twist. They are in blind holes so I don't want to chew them up, as it will require the removal of the complete block (approximately 1 ton) to drill them out. Any ideas will be most appreciated.
Swap the oxy-torch tip for a cutting torch and burn them out. An oxyacetylene cutting torch doesn't cut cast iron. This technique demands that the cut be made fast because if the cast gets too hot the top edge of the threads will melt. I find that it helps to drill out as much as possible, then use the cutting torch to make fast work of what remains. A quick run through with a tap will remove any leftover slag and bits of uncut thread. Practice on a junked engine in order to build up your confidence and courage.
Drill as best you can in center of broken stud. Ease hole out to where you're just short of cutting into thread. Pick a sliver of the old stud from the threads and try to peel it out. This might give a start thread for a tap (bottoming type), but it's gotta be straight. Gradually take the tap in, just a few degrees at a time, then reverse to clear swarf. Use lots of fluid, go real easy, try to dig swarf out from bottom of hole. This worked for me, but the important thing would be to not go too far without clearing the thread, otherwise you might bust the hole out due to excess pressure.
We used a method called 'air arc' (it used an electric arc and air to blow out the molten metal) to remove broken taps from cast iron assemblies.
Another process that our machinist used was the use of harden drill guides. These guides would be inserted into the matching stud hole and a small drill would be used to bore a center hole into the stud. Then slowly increase the internal size of the drill guide and drill until a tap could be used to clean out the threads.
Place a nut over the stud that is broken so that you can see the broken stud end through the hole in the nut. Using an arc welder, weld the nut to the stud, welding through the hole in the nut to the stud below. The stud gets VERY hot and you put something on it you can get a grip on. Put the wrench to it as soon as you can safely put the welder down and turn it off; it usually turns right out.
Use the flame wrench to burn the stud out. It works best if it's not a blind hole, but I've done it both ways-BE CAREFUL! You are blowing against molten metal that has no place to escape to except back at you. I've had best success if you can drill down the middle so that you can work the torch flame down the hole you drill and work outward toward the threads.
You'll melt the steel right out of that cast iron. Both of the above require a steady hand and patience!!
Broken bolts get the drill and cutting torch treatment. Pressing a wet rag on the near red hot joint forces steam under tremendous pressure through it too.
I also suspect that chills the stud more rapidly than the surrounding iron, causing it to contract and mechanically break loose from the hole around it.
Be cautious when using the cutting torch on cast iron, if you get it too hot the iron will become as hard as the tap and will be very hard to deal with. My first method in a case like this is to use a pair of vise grip pliers and a torch. Lock the pliers on the stud and heat the manifold around the stud. Start shaking the pliers back and forth (patience is a virtue), until you feel a creaking. Keep heating and shaking until it starts turning. If it gets too cool you will know, the bolt will lock up. If it breaks off I resort to drilling. I drill until close to the threads, and use what I call a bushing punch, cape chisel?, to peel the remains out. This usually works best if you are just a little off center, drill until you are hitting the threads on one side, the other side will have a little crescent that you can get the chisel under.
And the result:
From the number of replies I got, ATIS members have had lots of practice on removing broken and frozen studs. I have evaluated the methods that I used, not as a professional mechanic, just an enthusiastic amateur. The studs came out by a variety of the methods suggested-two with a stud remover and some bad language, three by drilling and picking out the remaining threads with a scriber, and four of them I blew out with the oxy and am now digging out the oxide with a tap and a scriber. Welding the nut on didn't work in this case, as the studs were so thin that I twisted them off. These are my conclusions:
1) Just heating the stud to red heat and allowing it to cool is sometimes enough to crack the rust and allow it to screw out.
2) Welding a nut on the stud and turning it back and forth seems to break the rust to powder and allows it to come out. In my case the studs were so thin it broke them off.
3) Drilling out is the next method. Taking care to center the punch will save a lot of work later digging out threads from an off center hole. I center drilled another stud 1/8' in the lathe and screwed it onto the frozen one with a nut as a drilling bush.
4) The final solution is blowing it out with the oxy torch. This is a last resort, as it is possible to melt the cast iron and destroy the threads. It also makes the cast very hard so that it is difficult to cut an oversize thread.
Drilling a hole through the stud first gives a thin edge to start the torch on and allows you to burn the thing out quickly without overheating the cast. Digging out the old oxide is not easy-I used an old Phillips screwdriver sharpened to a point and a small hammer.
Now that I have had all this practice, the one thing I have learned is to avoid this job if at all possible.