I don't know Medeco locks. It sounds like a mortise type lock.
Remove the scalp plate on the edge of the door - 2 VERY small
screws, be very careful as you remove them. Remove the scalp
plate. There will probably be 2 screw heads side by side exposed
that would be dead center of the cylinder. There is a groove cut
in the side of the cylinder that the screw closest to the outside
engages. The other screw is for a door that has the cylinder on
the other side of the door. If the screw you need to use is
missing, just steal the other one. Put a key part way into the
cylinder so you can turn it back and forth, start to tighten the
outside screw as you wiggle the cylinder until you feel the
groove, and tighten the screw. Put it back together and pray you
didn't lose one of those scalp plate screws. The cylinder's
groove is positioned so the teeth on the key will be in the up
Keep the whole world singing . . . .
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