CO-102 is a quick course on how to remove a STUCK THERMOSTAT COVER.

>"The thermostat housing will not come off. I've removed the nuts holding it in place, tried gently prying it up from the edges, but it won't budge. Any tips for removal?"

Start with a good soaking of the nuts and studs with penetrating oil. Remove the nuts in any way necessary. If the nuts come off without breaking the studs, soak it some more in penetrating oil, double nut the studs, and see if you can unscrew the studs that way. If you can manage to unscrew the studs you're way ahead of the game, but don't get your hopes up. Two of the three holes in the head are tapped through into the water jacket, and if the last DPM didn't use thread sealant in those holes you very likely have lots of rust around the studs filling the space in clearance holes in the aluminum housing.

If you break the studs off, don't worry about it, new studs are cheap and they probably needed replacing anyway. Soak more penetrating oil into the space around the perimeter of the studs. Find an old sharp blade kitchen knife or a pocket knife with a stout blade. Insert the sharp edge of the blade under the edge of the thermostat housing and pound it into the gasket space by hammering on the back edge of the blade. Do this on at least two sides of the housing. The knife blade will be substantially thicker than the compressed gasket, so it is pretty much guaranteed to lift the housing at least a little, just like a log splitter wedge. Once it moves a little you might hammer the housing back down (gently so not to break it), use more penetrating oil on the studs, and repeat the process as necessary to loosen the beast enough to be removed. If that doesn't do it you can insert a thicker knife blade, a flat blade screwdriver or a chisel point underneath and hammer that home to lift the housing more. Try to lift about equally all around so it doesn't get cocked on the studs.

When you do get it removed the bottom of the aluminum housing may be dinged up a bit where it mates with the gasket. To fix this use a piece of medium grit emery paper, lay the paper grit face up on a table top, oil it up to help carry away sanding dust, and rub the housing firmly on the emery paper until you get a smooth buff finish on the bottom gasket surface.

If the studs aren't out yet, soak with more penetrating oil and remove by tightly double nutting, or us a large locking pliers (ViceGrip) as necessary. Do try very hard not to break off the stud flush with the surface of the head. If you do that, the next move is to grind the stud flat on top, then carefully center punch the exact center of the stud, and proceed to drill a small hole vertically through the exact center of the stud (as best as you can keep it centered). Then use progressively larger drill bits (in small steps) until you get very near the root diameter of the female thread in the head. Then you can use a tap to clean out the remainder of the old stud from the threads in the head. Nothing particularly magic about this, and anyone can do it, but it may require some patience and persistence.