Go to the Midway USA web site, look up Wolff Spring Kits in the Gunsmithing area, all you need is one for a J-Frame, it'll fit all of the J-Frame revolvers.
I put a Wolff spring kind in my 642, 8# hammer, 13# rebound and it works great. Just make sure to polish up the part that the rebound spring goes into, you need to take a stone to both sides and the bottom, you can also polish up the DA sears while you're in there if you like. Don't touch the SA sear on the hammer though, they're already pretty good and you don't want to mess it up. Good luck.

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S&W M642-2,
ATI 1911a1, 4.25"