I've got a dilema for ya folks. I tore down the top of the wifes car to refresh all the gaskets and adjust things before I changed out the tranny. With everything out of the way, I noticed the rubber in the balancer protruding and breaking off. No sweat. Time to change it, I've got one. I put a 5/8" 6 point impact socket on my air gun and let'er rip. It impacted for a few seconds and then spun free . All of the sudden, I remember when I put that motor in I saw that issue and experienced that rounded  bolt once before. The 1/4" washer behind it was actually sunk in the middle as if someone used a "badder" air tool than I have. Anxious to get'er running I put off this battle 'til later . Well, later doesn't have to be now either but with all this room to work I'm hoping to arm myself with enough ideas to do battle with this thing and win. Lord knows I don't want to have to change the crank (too bad, her vette was running so good, I'm sure she won't understand what the He** I'm doing to it or why ). I have already tried a nut extractor, with the tapered teeth that bite. I used my breaker bar and it grabbed and then spun. I 've searched the part houses for miracle tools, but they only stock what I already have. I've been advised to weld a nut on to it, or cut the bolt head off and remove the balancer and then grab the shoulder and remove it but, I'd like to collect more ideas before I do anything I might further regret. Any and all ideas are welcome my friends. I've got the radiator and all out, so I guess I'll put the top end back together while I ponder this developement.  aceinthehole2010-05-15 16:35:51

that washer is pertty thick to be sunken, you have all the ideas that I could suggest, good luck

You may wind up cutting the head off, and then welding a nut on the end. I've seen a couple that were really tight like that, but....I've seen more that stripped the threads in the crank from being too tight...
There's really no need for a crank bolt to be that tight....yeeesh! Bubba been around yer car, has he?
Another off the wall idea is to try a pipe wrench on it. I'm assuming you already have all of the pulleys off. You'll need some way to "lock" the engine down so the crank won't turn...easiest to lock the flywheel, or if it's a standard trans, disconnect the shift linkage, and put the trans in Rev and first at the same time. That will lock the crank down.
One trick I've used many times, tho it's a little dangerous, is to put a socket with a long breaker bar on the bolt, and wedge the breaker bar against the frame. Dis-connect the ignition wiring so it won't start, and then "bump" the key a time or two. This makes the starter break the bolt loose. Of course, if the bolt head is messed up, this ain't gonna work, but if your pipe wrench is long enough, you can certainly give it a try...just be very careful.

being a certified navy welder at one time I personally would not use a torch to cut the head off, too confined an area and you might damage the lower pulley or send a hot one somewhere and cause a fire, I think I would grind the head off, after removing the dampner you will have the stub left, I don't think the bolt is long enough to bottom in the crank it might come out with little effort, if some bonehead used lock tight then I would heat the stub and then let it cool stone cold, what happens is the stub expands when hot  and retracts when cold hopefully that might loosen any lock tight, if you heat the stub it will expand and makeit harder to get it out, if anything you want the stub cold and the crank snout warm, and if that doesnt work weld a nut onto the stub and say your prayers.

I once welded a small spark plug socket over a bolt after I wiz-wheeled the stripped head off. The socket has a six point end on it (thing it might be 3/4"). Used a breaker bar with an impact socket on that and presto!! Bolt came free!!  lukesvette2010-05-22 19:32:57