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Probably hard to get a drill in there, but left-hand drill bits are handy for jobs like this. Since the bolt head is already a hole, you can skip making a pilot hole as long as you have a large enough left-hand drill bit (or an Ez-Out).

 

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regular hammer and a chisel worked for me if you don't have an air compressor/chisel

just make sure you hammer on the side that will turn the bolt counter clockwise (it's a regular thread)
 

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Oil and Heat will break any nut loose.

The head is probably stripped for a reason/reasons. Some sort of oily spray, like WD40, applied liberally along with heat with remove almost any nut/bolt. Of course high quality tools are helpful, nothing worse than stripping allen head from using a harbor freight wrench.

The next size up comment is pretty much standard operating procedure. Sometime it's worth it to bang in the next size up. Along with some oil, hammer, heat, air chisel, drill, bigger hammer ok ok I'm kidding.....kinda.

I'd consider drilling a "last resort" type of action, but not out of the question.

Good luck with it.
 

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The head is probably stripped for a reason/reasons. Some sort of oily spray, like WD40, applied liberally along with heat with remove almost any nut/bolt. Of course high quality tools are helpful, nothing worse than stripping allen head from using a harbor freight wrench.

The next size up comment is pretty much standard operating procedure. Sometime it's worth it to bang in the next size up. Along with some oil, hammer, heat, air chisel, drill, bigger hammer ok ok I'm kidding.....kinda.

I'd consider drilling a "last resort" type of action, but not out of the question.

Good luck with it.
Good points. I'd consider the Easy Out method a non-starter. If the bolt is tight enough to round out the head, then an Easy Out is likely to break off inside the bolt, leaving you with an even bigger mess. Heat, plus a larger hex key pounded in, followed by a chisel,either manual or air, would be my first choices, followed by drilling the head off if all else fails.
 

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It probably wouldn't work here, but guys who have welders can hold a large nut against a stripped bolt head or broken stud, and weld on the nut (by filling the center with weld) to create a new head on the bolt. The heat from welding also helps loosen the threads, but don't try to remove the "new bolt" until the weld has cooled.
 

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Vince1912

The pic you show is zoomed in to show the head of the SHCS (Socket Head Cap Screw) down in a c-bore. This doesn't look like the screw mating the fan to the visco-clutch, but maybe I'm mistaken since I can't see it in my W140.

For any screw with a broached internal drive drive engagement, it is not too hard a job to drill into the broached socket (it's on-axis and provides a good means to locate the drill). This is assuming that you have full access to the head of the bolt/screw.

It can be drilled somewhat step-wise in sizing up to a size approaching the major diameter (size of the screw). This is usually a bit larger in diam than the broached socket. If you have some of the screws out, then you can estimate the drill depth at which the head will come off when drilled to the major diam of the screw. You know the diameter & you know the depth, so it's just a matter of doing the drilling carefully. A caliper would be nice just to be sure about all sizing; drill, bolt, etc.

You don't need left hand drills. Bolt is likely stripped due to the preload on the bolt rather than the threads being seized. Once the head is off, the preload is zero and you will have the cylindrical shank of the bolt sticking out by the thickness of matl that the bolt was holding down. Usually this is enough to grab on to and turn the bolt with enough torque to get it out, using PBlaster and any other tricks.
 
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