Scared to loosen 18 year old thermostat bolts!

[abscate]

Please don't hit it with force. Changing the housing is no fun.

+1000

Light taps vibrate the fastener and let PB work in and do it s magic. Just let a small hammer fall under its own weight.

[98V70T5]

I see why using an impact driver would have been a very bad idea!

[JimBee]

Doing this little task should be easy, but as rspi points out, if things don't go well, you can end up changing the whole T-stat housing, which in the worst case, can require removing the cylinder head.

In the two 850's where I couldn't get the bolts out, I didn't have a brand new high-end Torx driver, though mine were fairly new. The points on the screws had already been dulled a bit when I obtained the cars, so that made success more iffy. The other thing I didn't do was douse the bolts daily for a week; maybe that would have made them removable. I cleaned out the Torx head with an awl, definitely a good idea.

As I have written up here long ago, I drilled off the heads and used the two-bolt method to "grab" the screw shanks and turn them out. On my 93 850, one of the screws squeaked and groaned until it was almost out. I would definitely try the daily-for-a-week PB Blaster dousing method if I had to do this again. Another good idea.

Robert got lucky being able to turn out that screw shank with pliers.
Assuming the PB Blaster doesn't loosen them enough to turn out with the Torx bit or the head strips out, your only option really is to drill off the head.
I've done 4 bolts that way, using a drill bit of high quality steel that is larger than the bolt shank diameter, almost as big as the head, then drill the head to the point where you've reached the shank. When you get to the top of the shank, there will be no remains of the head, but you will have the full length of the shank to work with. AND where the top of the shank met the head, the threads will be cleanly shorn, which will be very useful.

That's important because if the shank threads are still corroded into the housing (despite using the PB Blaster for a week) not even a vice grip on the threads will turn the shank out. AND if you try pliers or a vice grip and mess up the threads on the upper most 3/8 to 1/2 inch of the shank, you might not be able to get it out at all. I didn't hit this brick wall, but if I did, I would use more PB Blaster on the exposed shanks and let that soak in for a day or two. If that all fails, and the upper end threads are messed up, you'll need to try to remove the whole housing from the cylinder head. There you can run into the same problem, which might even be more likely.

The two bolt method. This is the best way to remove stubborn bolt shanks and really quite easy if you have drilled straight down and used a bit slightly larger than the shank, because the top threads will likely be clean. NOW, with the T-stat cap removed, you can thread a nut onto the frozen shank far enough down to be able to get a second nut on top of the first one.
Align the flat sides of the two nuts, slip a box wrench onto the lower nut to hold it and tighten down the top nut to the lower one—firmly, so they are locked together. You don't need massive torque—you don't want to strip the shank at this point—but they do need to be tight. If the nuts are not perfectly aligned, you can leave the wrench on the lower one (or just let it fall below the nuts), as you won't need it for the next step.
Now, use your socket wrench on the top nut (or over both if they are aligned) to gently begin turning out the shank. It WILL release. I recall at least one shank squeaking and groaning just about all the way. Yes, I used antiseize on the replacement hex bolts used to put it back together.

The key to the two nut method is really cleanly drilling off the bolt head, as I described then carefully threading on the first nut, maybe working it a bit up and down the top threads to get the second one on. For stubborn/frozen shanks I believe it's the best fail-safe method for getting them out.

[JimBee]

Edit: next to last paragraph should begin Two nut method.

[sleddriver]

My soln was to buy a long-shaft Torx screwdriver with a flat on the handle. Even then I bent the male driver end! Used AL anti-sieze upon reinstallation. No worries since.

How did you get leverage using a screwdriver?

That's why I bought a driver with flats on the handle. So I could add torque with a large crescent wrench. I was surprised it cork-screwed the Torx male end. I thought the steel would be harder than that, especially since I paid for "US made".

[Knowprblm 97]

I just finished my thermostat on a 1997 850 glt and I stripped the back Tork Screw. The front came out fairly easy after letting them soak for about an hour in PB Blaster. I used the tighten and loosen method that that's recquired for removing the spark plugs on the Ford Explorer engines. Once you here it crack stop and add more lubricant and wait. The next thing you do is turn it in the opposite direction, stop add another shot of lubricant and wait. Go back and forth til it comes out.
On the back bolt the tork was not at a good angle to seat properly, and it was pretty worn to begin with. After I drilled the head off the torx bolt and removed the thermostat and housing, I couldn't believe how dry the stud was. It was as if no lubricacnt had made its way past the head of the bolt! So I hit the stud with some more PB Blaster waited and then I hammered a 6 mm socket over the 7mm stud using a long extension. Once it was seated I just added the ratchet to the top and spun the stud out.

[mnorris23]

I would PB blast the crap out of it then carefully try and break it loose. Personally I didn't have an issue removing mine to replace my thermostat on my 17 year old car.