From 
Image 1 of 4
1. Probe the
existing
hole with a keyhole saw to
find the horizontal edges of the lath. Center the box on the lath
and draw its outline. Cut down on
both sides of the center
lath and remove it.
Image 2 of 4
2. Support the top
and
bottom lath with your
fingers while you cut out about half of each to fit the rest of
the box profile. Push the wires to the
side so you don’t
nick them.
Image 3 of 4
3. Cutting the hole
accurately is critical
so that the box ears have
solid bearing on the plaster. Partially cut lath will support the
top and bottom.
Image 4 of 4
4. Insert the wires
through the box clamps
and insert the new box.
Slip metal box supports along each side and bend the arms around
the box edges to anchor it solidly.
Mount the fixture.
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