Annealing Steel / Broken Screw in Block

I've got an old Mighty Mac shredder/grinder, with an 8 horse Tecumseh engine. Aluminum block.

Noticed that the intake manifold was coming loose, and had to remove the muffler to get at the screws. Muffler was held on by two long 5/16 - 18 hex head screws which pass through spacers inside the muffler. One came loose after PB Blaster, waiting, and some pretty healthy wrenching. The other snapped off a couple of threads proud of the block. It's got maybe 1/2" of screw in the block.

Tried to drill the broken piece out, using a standard HSS 1/8" bit. Wanted to make a centered pilot hole, then drill with a larger bit, try Easy-Out, then most likely drill larger and pick out the threads. Got in maybe 1/8" or so and it started to squeal. Then tried another bit, which I used Tap-Free with. Instant squeal and no chips. Think I must have work hardened it. It doesn't look like stainless steel as it's got some rust, but I suppose it could be. Or maybe just some work-hardening alloy steel.

1. Can I use an O/A torch to anneal the screw? Any worry about the aluminum block?

2. Any chance a cobalt bit would cut it, or are they only slightly harder? I'm guessing carbide would have no problem with it even in the work-hardened state, but I don't have any carbide bits. I've been drilling with a hand-held drill - with carbide I'd probably have to mount the engine on the horizontal mill to drill it. I could do that, but I'm hoping there is an easier way. Mounting it, and drilling on the backside of the work, would be a PITA. Plus, I'd have to find some carbide drills. Straight flute the best for this?

3. I know that alum, nitric acid and some proprietary products will remove steel taps without hurting aluminum. Since this is a screw, however, with no flutes to allow the fluid to work along the length of the threads, I'm thinking it would take forever. If I am able to drill it, and then drill larger until I hit some threads, that would speed things up and might be an option. I'm still not sure, though, that the screw might not be stainless. If it is, do the chemicals work on stainless?

4. I could take the engine down to the local machine shop, and have him try the "weld on a washer and nut" trick, but I'd rather not mess up the pilot hole I've started if it might be of some use, as it's well-centered.

I've got a couple of tricks for centering holes on broken studs or screws that I haven't seen others use, although many of you may use them. If the screw is broken off down in the hole with a few threads showing, I'll drill a hole through a new screw on a lathe, then thread that new screw into the hole and use it to center the drill. Works well even if the break is jagged and angled. If the screw is broken above the hole, as this one is, I'll do the same thing - except that I'll also tap out a piece of hex stock, face it on one end, and screw it down on the remaining threads, like a coupler nut. Run the drilled screw down through it until it hits the broken piece, and the pilot hole will be on center. If the screw is broken off flush I clamp on a plate to guide the drill, but the center location is then a guess.

4. I don't think the local shop has an EDM machine, but I could probably find one that does. Do those remove broken screws as well as they do taps? If so, any idea of the typical cost?

5. If I can anneal it, should I again try the pilot hole and then open up larger method, or drill to final size in a single pass? I like the pilot hole method to keep the hole centered, but if it's going to work harden again and break the corners off my larger drills, I'll do it in one pass. I still like the idea of trying the Easy-Out and drilling larger and picking out the threads only if (likely) the Easy-Out fails, but if the Easy-Out has a chance....

6. Any other thoughts?

Thanks,

John Martin

Go buy a cheap masonry bit with a carbide tip. Take a diamond stone,even a small fish hook stone will do,and sharpen the bit up some. Run the bit as fast as your drill will go. It will drill out a hardened bolt,but the problem may be you trying to put enough pressure on the bit to force it to cut. A drill press would be better,but may not be practical. If the bit can't be pushed hard enough,can you put a bar clamp on the butt end of the drill to give leverage,tightening the screw as you drill? Or,get another guy to help push. If you get the bit really hot,the brazed tip can come off,but without a drill press,I doubt this will happen.

If it were me I would plan on drilling the screw out and also fixing the threads with a Heli-Coil. Usually when things are that tough to remove, it means that the threads are damaged anyway and have seized onto the bolt.

As far as a the hardness of the bolt goes, it is likely hard either by design or by the heat of the exhaust. In either case you will have a tough time of it. Drilling out something like that by hand is a tough job. To me it would be much easier and less error prone if put on some sort of drillpress or mill. I had a set screw that was cracked lengthwise once. That was a bunch of no fun even with a drill press.

I have seen some bolts that were tough at the outside but soft and chewy in the center. One I got a 1/16" drill down the center, I was able to coax the rest out. Left hand drill bits can also help.

Good cobalt bits will work better than HSS in tough materials.

I would try welding a nut to the stud. Preheat with a torch first...the heat will transfer to the aluminum which should cause it to grow...more. Don't be afraid to "fill" the nut up with weld, I'd go for a 3/8-16 nut or even a 7/16 if available.

Even if you snap off what's left flush you'll have a fresh surface to work with, and less distance to drill.

The hardest bolts I ever had to drill were on the exhaust of a 3406 Cat. I could get a good weld on them, but it wouldn't unscrew them. And everytime I welded to them I had to grind the stud with a carbide burr before I could drill them. I have had other poor experiences with exhaust studs, but never that bad. I don't use the welding technique on exhaust side breaks any more.

If the hole is well centered, I would give it a shot on the horizontal. Maybe 25 sfm with hss.

If this were my Mighty Mac (It isn't, because mine has a Briggs engine) I'd assume that the threads are going to be galled and pulled apart even if you get the stud out, and I'd take my smallest oxy acetylene welding tip, set it to a hissing little oxidizing flame, and fizz the bolt out. It will mess up the aluminum a little, but those exhaust studs aren't all that critical anyway. It usually leaves enough thread to function, and if not you can helicoil it, or even just tap it one size larger.

I forgot to add that of course,after you drill out the stud,you use an easy-out. Just do what I said and you will be allright.

Use kroil then put a nail punch in the drill hole and tap it for a few minutes.
then take a steel rod that will just fit into the drill hole and heat it red hot and put it in the drill hole till it hits bottom. do this about a 12 to 24 times.
after that take a a can of dustoff or any compressed gas duster that contains difuoroethane and use the little tube and turn the can up side down and freeze the stud

Then while it is cold take a drill motor and set it on rev and use a left hand drill bit.
Many times the stud will unscrew right out as the drill bit digs in.

Whenever i have to drill out a stud i always use the left hand drill trick.
Most of the time it comes out before i have a chance to use the easy out.

I use to restore 75 to 100 year old mining equipment.

You can also use a sacrificial right-hand drill running in reverse to spot-anneal it. Used to use a steel rod in a drill press to spot-anneal mauser receivers.