These small seized screws are a real challenge, to say the least.
In my experience, I've broken a couple of miniature screws while attempting to remove them -- the heads sheared off when I applied excessive torque. In one case, I attacked the seized screw with a JIS driver and a mini Vise-Grip. In another instance, I had to drill out a couple of screws on a lens mount (Pentax-M lenses are notorious for tight screws on the mount). I ended up drilling out the screw heads but consequently enlarged the tapered screw holes on the mount -- the mount was damaged beyond repair so the non-exotic lens is now for parts.
If the head shears, it's a difficult-to-impossible job to drill out just the shaft or remove it with a micro extractor. If such a job is successful, re-tapping the hole might be necessary.
I would recommend two approaches:
- Continue with small amounts of Kroil for a few more days as you suggested. Patience is the key. (Beg forgiveness for occupying the living room and the tapping sound?
)
- Apply heat to the screw head with a low-wattage soldering iron and pencil tip, if it seems reasonably safe at the subject location on the camera. Of course, watch out for non-metallic sections nearby.
- Craig