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John Coffey writes:
In my opinion butterfly keys are a waste of time
as once they shrink (and wood is always moving)
and the top shrinks all the angles of the mortise and
the butterfly will change and the butterfly will push
the wood away (because the mortise is shorter).
The tightly mating surfaces will only be tight
when the crack actually opens.
Plywood butterflies would be better
but once the top starts to move
all those angles start to change.
Shrinkage is exaggerated for illustrative purposes.
If you don't want to rejoin the tops (my preference),
then what I usually do is insert a shim and glue in with epoxy.
On splits wider than 1/32" I might open up the crack with
a crack tool, a file tang, bent over, hardened and sharpened.
I then open the underside (non-show) and epoxy in a shim
with the matching tapers, then clamp the top to make the
crack on the show side as small as possible.
This is great for splits as well as glue line failures.
I will put a clamp (or weights or go-bars)
on the crack to align the top surfaces.
John Coffey
Locust Valley, NY