I don't know how to explain this, not really knowing the name of the parts but.

Using the schematics I got from Brownell's I'll use the numbers.

There is a paw that indexes the cylinder when the trigger is pulled. (27). The paw has two pins, one to fit in the trigger and the second, shorter pin holds the spring so it pushes the paw into a slot to index the cylinder.

The shorter pin on my 642 is too short, it allows the spring to slip by prohibiting the paw from being pushed into the slot so it can index the cylinder. (spring is 30).

Now do I fix this? The smaller pin seems to be pressed into the trigger body, can I use a small punch and push it further in so as the spring can't slip by?

It sounds like your talking about the "hand". If your having troubles and your revolver was made after 1994 I believe it's covered under warranty.

Replacing the hand is for a gunsmith not a novice, I'd give Smith a call and have them fix your gun.