I've got a Valley pivot gearbox that the bottom outside bolt sheared off on.  I'd like to fix the thing without removing the gearbox.  I tried some left hand drill bits with no success and then tried some old easy outs from my grandpa.  They bite just fine into the old bolt but I'm unable to get a good grip on the easy out.  I've already drilled a 19/64 hole in the 3/8 bolt.  Am I wrong to weld 1/2 nut onto the easy out to give me something to grab on to?  Can I safely heat up the gear box without damaging it?  If I heat it should I leave the oil in it?  Change it when done?