The two changes I made on my new Model 60 were the grips and the springs. I went with Pachmayr Diamond Pro grips which gave me some added length so all my finger fit and also were thicker which felt better in my hand. I think they have these for the 640.
As for springs, I added the Wolff springs (hammer and rebound). A fairly easy install. I did buy the SmithMaster tool to help install the rebound spring. While I don't have a gauge to measure, the trigger is lighter and much smoother.
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