1. What size drill bit would I need to drill a hole for the screw extractor? And will it be strong enough to do the job?
There are only several sizes of extractors and normally the recommended bit to use is stamped on the the extractor. Use an extractor that is about half the size of the screw shank.
2. As a second alternative, if I drill the hole right through to tear the broken aluminum screws apart, will it go through the engine? Or is the spacing for the valve cover screws located outside of the engine?
You want to try to limit the depth of hole you drill as you want to limit the drill shavings in there. The screws do not go right through to the blocks but to the flange/lip of the cylinder head; do not drill through as you might ruin the mounting boss. Drill just enough to where you get a "bite" - slowly turn as you lightly tap on the extractor. Replace with same screw material (AL); otherwise you will have issues with galvanic corrosion (dissimilar metals under heat/stress). Torque per spec.
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Last edited by Nube1kenobi; 09-12-2010 at 10:43 AM.