I’m trying to get a bolt loose on the latch fitting on my car’s trunk lid to adjust it. The bolt has a “star” hole in its head, similar to an allen head; I got a set of wrenches for that kind of bolt.
The bolt, however, is stubborn. It is almost flat, and there isn’t a lot of (clear) space around it. I tried twisting it out with a needle-nose Vise-Grip, but it’s hard to get the narrow jaws to stay clamped to such a small surface. The star hole seems to be stripped…
pictures might help, of the bolt head and the area that its bolting onto? If the star-pad is stripped, try using a flat-head screw driver and a hammer to turn it.
Put the screw-driver against the outside perimeter of the bolt head (sounds like it would be easier than trying to get it into the star-pad opening) and tap it (somewhat gently) with the hammer to turn the bolt that way.
If there’s some star left you can try putting a little aluminum foil over the head and forcing a bit into it. A screw extractor (reverse thread type bit) will take that out with a some minimal drilling into the head.
Probably a Torx.
You can buy a tool to remove these when they strip. Like this. Basically you drill a hole in the centre and then use the tool, which grips the hole you drilled and allows you to use a spanner to apply force.
£3.99? How much is that in dollars and cents? And where would I be buying it from?
Hardware will have them.
I’ve just spent a couple of days removing very rusted large screws from old outdoor furniture, and I’ve broken more drills and screw extractors doing so than in the entire rest of my life. Don’t buy the exact set shown in bob++'s link though; I have (had) that exact set and they are next to useless. Screw extractors are one of those things that are being asked to do a very tough job and only just work - at the very edge of the capabilities of steel’s hardness and toughness - at the best of times. Shell out for the best quality you can afford.
I’d start with some penetrating oil, WD40 or whatever is common in your area. Let it sit for a day, spray it 2 or 3 times, then try to use the Torx driver again. I have easy outs and left handed drill sets, but I seldom need them. Sometimes a screwdriver with a vise grip on the shank of the driver, helps. That way you can push real hard, and turn the driver hard.
+1 on penetrating oil, but WD40 isn’t really ideal it’s more general purpose.
Kano Kroil is a good penetrating oil, also it smells nice. 
http://www.kanolabs.com/msn/
Professional mechanic here. Use the penetrating oil, wait about 5 min then rap the head of the screw sharply with the smallest ball being hammer you can find. 2 or 3 sharp raps and then try again with your bit. You can also lightly tap the end of your driver while holding pressure against the screw. Don’t hit the top of the screw too hard just a quick little rap.
Ball peen (sometimes, pein) hammer, I reckon.
I’ve often had success by heating a stuck bolt with a torch (for extra effect, shock-cool it with water). Of course it needs to be in a location where a torch can be safely used.
Former Army mechanic. Go with the percussive shock and the penetrating oil. Heat does work, but be careful with it if the thing is dripping with penetrating oil.
If it’s small, you can just drill the center of it out, then pull it out with pliers, but make sure you have another one before you destroy it.
Well, that should be easy to find. I think this one is shot anyway.
Really, any type of hammer will do.
Another possibility is to take a small, sharp steel chisel (not wood) and tap the edge of the bolt in on it’s rim/edge in the direction to loosen it.
Yes, I’d just go straight for this. My experience working with these types of body fasteners is that they’re made out of tissue paper and you’ll twist the head off before you actually get the threads out. You need to apply strong inward pressure on your bit driver the first time and hope for the best, but when they inevitably strip out I just grab the drill and twist out the rest with vice grips.
Sometimes it’s just easiest to drill the old bolt out, tap new threads, and use a new (larger) bolt.
If it’s not out by this point I’d break out the angle grinder and oxy-acetylene torch and turn that thing into an Animal House Deathmobile.
I’ve had luck with…
-Hammering.
-Heating.
-Impact air powered hammer wrench.
-Vice grips around outside of bolt.
-If all else fails, drill it out and replace bolt.
Step One: **Use the correct tool. **
The recess in those auto body screws isn’t anything like an Allen. Dollars to donuts, it’s a Torx. Using the correct Torx driver will deliver a lot of surface area that won’t slip or “cam out,” but using anything “close enough” will just make life difficult.
Too late for Step One? Sucks to be you. You may be able to hammer a soon-to-be disposable screwdriver into the head and turn it out.
If it’s safe to do so, another trick is to weld a nut onto the butchered bolt, but I suspect there’s not enough clearance for the OP unless they’re a TIG ninja.
My go-to penetrant is PB Blaster.
One of the best ways I’ve found of moving a stuck screw is by cutting a slot across the head to use a flat bladed screwdriver in, using a spanner/ vice grips if necessary, as mentioned above.
There are a few ways to make the slot, depending on how recessed the screw head is. Angle grinders are usually quickest, if you have room and a steady hand, or a dremel type tool with a cutting disc in. If this isn’t workable, simply hitting it a few times with a good, sharp cold chisel can often work, once a dent is made some pressure can be put on with vice grips as you keep tapping it.
I’ve managed to use a centre punch to tap the head of a screw around many a time, this is my preferred method to extract rounded countersunk bolts, once you have moved it a turn and a half or so, pliers can be used to take it the rest of the way.
I’m surprised no-one’s mentioned freeze-release sprays yet, they can work wonders if the threads have seized with rust, given multiple applications and maybe an hour or so to let it soak through. My favourite is Wurth’s rost-off ice, but there are plenty of brands to choose from.