Remove heatsink screws?

[Chuck H]

Heating the screw will make it expand which might make things worse.
I'd try to mainly heat the surrounding metal (heat gun) and then hit the screw with freeze spray to make it contract.

I've done exactly this for automotive work. With mixed results. Thing is... If you heat the surrounding metal it too expands. In all directions. Reducing rather than expanding the diameter of the female threaded portion.

[tedmich]

On cars Ive always liked a combo a striking the fastener straight down with a hammer (use a pin punch if small). If that doesnt work heat the part gently with a torch and then apply a dry ice chunk just to the head for 10-30 sec. before breaking it.

These can help too:

which combine the axial blow with a rotation, this one uses 1/4 bits (only impact rated!) while the japanese versions of these are popular for motorcycles they use an odd large hex size bit.

[Mick Bailey]

Almost all.the vehicles I work on have seized fasteners and striking them can work wonders. My father used to have those screwdrivers with a through blade and striking head on the handle - made with this in mind.

The idea of striking the fastener is to break the bond. The same with heating or cooling. Whilst heating the fastener appears to be counter intuitive, it really does work. With button head screws the surface area of the head is large compared to the thread diameter. Heat applied to the head expands it outwards and away from the parent metal, reducing the friction at that point. With dissimilar metals such as a steel screw in aluminium, the aluminium has a much greater coefficient of expansion than steel. The metal around a hole expands as though it's a straight line, so the hole increases in size slightly.

Heath also breaks down adhesives, thread lockers and general gum.

Due to the number of stuck fasteners I encounter, I put money into a low torque impact gun. This has to be seen to be properly appreciated. There's almost no torque to speak of, but the high frequency vibration applied for sometimes several minutes undoes things like badly corroded bleed nipples in brake calipers, diesel glow plugs etc, that snap easily. I just had a plastic motorcycle mudguard that had four M8 button head screws holding it into alloy. Completely corroded and no chance of heating and three had rounded out heads. I have half a dozen types of release agents, but none worked. I used a Torx bit with the relief ground of, hammered into on the head and it took several minutes with the gun but they came out perfectly.

[Helmholtz]

If you heat the surrounding metal it too expands. In all directions. Reducing rather than expanding the diameter of the female threaded portion.

Interesting thought.
So you think that the hole actually reduces in diameter when the plate is heated?
If that's true, cooling the plate should do the opposite and work better, but:

The metal around a hole expands as though it's a straight line, so the hole increases in size slightly.

That's what I always thought.

[tedmich]

as a metallurgist explained it "During a thermal expansion or contraction, any hole in solid behaves as if it were full of the same material. Therefore the diameter of your hole will increase according to the expansion coefficient of the material."

Many high precision cutting tools are installed by heating the holder and a shrink fit on the chilled bit

[Mick Bailey]

Shrink fits are a popular way to retain bearings in old motorcycles. Heat the alloy crankcase, drop the bearing in and when it cools the bearing is held really tight. Same thing with cast iron valve guides an alloy heads in cars and bikes.