Prius C rear license plate bolt rusted

I bought one of these: http://www.harborfreight.com/screw-extractor-and-left-hand-drill-bit-combo-set-10-piece-63987.html But I didn't have much luck with what I was trying to remove. A drill press would make it easier but not possible without major disassembly.

If Prius C is same as Gen2 Prius the threads are on a removable clip that you can replace after you rip out the one with the busted nut in it.

 

If this were my job, I'd probably find a screw head that matched the other one and use liquid metal epoxy to hold the head and plate in place. It's not worth the hassle to do more than that.

 

I've fought many rusty bolts- don't forget the power of time. Hit it with penetrating oil, clamp it with vise-grips and use a driver at the same time. Try tightening as well as loosening. Even if you can only move it a couple of degrees, do it. Then give it a day and repeat.

Sometimes you can get some action by putting the driver in the screw head and giving the back of the screwdriver a sharp strike with a hammer, just to shock the threads a bit.

 

Lisle and Thexton sell swell tools for that kind of thing, if you have access to an air hammer:

lisle thexton fastener removal -plastic at DuckDuckGo

The hammer just gently (you feather the trigger) but rapidly bops straight down on the tool, while you easily turn it from the side (with a wrench on the Lisle, or the Thexton's own little handle). The air power is not used for twisting like an impact wrench, so you aren't going to break the head off or strip it. This thing has worked for me like magic.

If the search above comes up with a 9 minute 38 second youtube video, don't watch it. In 9 minutes and 38 seconds he actually never shows the tool in use. It takes him 5 minutes to get around to showing the tool at all. My neighbors don't often hear me yelling "edit your effing video" at my phone, but they did just now.

Of course the tool is intended for when the head is still there. If it's already cut off, this tool won't really be the thing.

 

The professional method:

Drill out the center of the Phillips screw with a 1/8 drill bit. Use cutting oil or any other lubricant.

Progressively increase the drill size until reaching the number 4 drill, which is the M6x1.0 root diameter.

Use an awl and a pair of needle nosed pliers to remove the remaining thread which is like a spiral helix wound wire.

Retap the hole with an M6x1.0 tap.

The alternative would be to drill out the bolt and hole for an M6x1.0 rivet nut.

 

Your first post mentioned the plate had been cut off. Is the stuck bolt head still there (even with the cross recess rounded out)? Or did it get cut off as well, leaving only a bolt shank ending close to the surface?

If there is a head left at all, I would probably still try the Lisle or Thexton thingy, just using a straight bit, and Dremeling a straight slot into the head for it. The magic of the tool is the straight bopping on the axis of the screw, not how much torque you can put on to turn it. What happens while it's being air-bopped is you can turn it relatively easily using the handle/wrench on the side of the tool.

That makes it a little bit different from the hand impact tools you bop with a hammer. Those contain a cam so that some of your bop is turned into torque. In the air version, all of the turning torque comes just from your hand on the handle/wrench, and all of the air bopping just shakes the screw on-axis.

If the head got cut off, I might at least pop off the inside trim and see what I could see. If there is a way to chomp some Vise-Grips down on the screw projecting inside, I might try to kind of replicate the air tool by doing that, and applying some torque with the Vise-Grips, while bopping on the cut-off outside end of the screw with a narrow punch chucked into the air hammer. (Again presuming availability of an air hammer.)

Getting the old screw cleanly out of the welded nut would still be my first preference, before I would go to a solution like Georgina's. Drilling out to the root diameter and retapping is great when the drilling is perfectly centered and everything goes right, and maybe things go perfectly for other people more often than they do for me....

 

Ah, yeah. Deadlines and rustbusting aren't great mates.

If you do decide to drill the thing out, you may want to do a prep step of using a dremel or the like to grind a flat face onto the screw, such that you can hit it with a center punch so that you have a nice clean landing for the first drill bit.

(I hate the thought of a drill skittering out of a screw head and then scratching the paint.)

Good luck!