Is the bolt still there rounded off? There are a couple approaches that can work before you need to drill it out which can be a nightmare - avoid that if at all possible.
They make tools called "stud estractors" which get tighter the harder you turn kind of like a pipe wrench. Better yet is to soak whatever you can get at for threads with PB (power blaster) and wait a day if needed. Find a socket metric or standard that you are willing to abuse. If you get one just close enough - hammer it over the rounded bolt till it's as tight as possible without breaking the socket. Best now to use a weak air tool. The 3/8ths speeder air ratchet for starters. The idea is to rattle the rust where it is stuck. This tool is generall not strong enough to break the bolt. Go forward a little and backward a little and when you detect any real movement of the bolt itself keep at that a little at a time - forward and backward (out) and eventually it should give way and come out.
Take all the time you need as it will cost tons of time if it breaks and lots more work. If it breaks off flush I don't know of a way other than drilling it out. Must be awesome strengh metal drill and a straight line in center of now stud. Screw extractors can be used and don't break them off as I don't know of a drill that can drill those!! Heat like oxy-acet can get just the bolt glowing but that should be a last ditch try. Heat takes away some of the hardness of the bolt (you've already welded it) and it will break off even easier. Heat expands which actually makes the bolt larger which is tigher but heat transfer to the threaded hole is slower and it tends to break that bond.
This is tough stuff and in some cases you may have to remove the part that is the problem to get a straight shot at it,
T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/
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