Replacing existing 9" dome fixtures with new 13" ones
Holes 6" apart for a lamp fixture are not very common. You need larger or adjustable crossbar (also called a fixture bracket)



You can attach this type of bracket to standard boxes with holes at 4" or 3 1/2".

Depending on whether the crossbar you find is threaded at 6", you may need to install machine screws in the crossbar slots, locked into place with nuts, that then stick down through your fixture mounting holes.

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