Since it holds the force of the spring. That is why it is a one-piece casting, but I am not sure if you were thinking to make them separate. Certainly there is a LOT of shear area in the factory piece that could not be easily replaced with fasteners. Let me know if I misunderstood what you were saying.
If you make the sear out of steel, then case harden it with something like this -
http://www.brownells.com/.aspx/pid=7626/Product/KASENIT - and figure out a way to attach it sufficiently to an aluminum piston, that would be the lightweight setup! Let us know how it goes!
Mike
