Is it possible and even worth it to have a ball detent put into the crane area of my 640 as more security?

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Generally the ball detent is added as part of an accuracy package to positively align the cylinder in the same spot every time.

As far as I know it does nothing for reliability.

I wanted it for more lock up security when firing 357 loads.

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I do have a 625 with a double detent. It takes substantially more effort to open it then any revolver I've seen..

It would provide a little stability, and would conceivably take a little stress off the ejector rod and center pin. I don't know what folks are charging to put one in nowadays, but I'd be inclined to think that it isn't worth it. I've put a couple of ball detents in revolvers, years ago, and recall thinking, "Why did I bother?" The original purpose of the concept was to provide some front lockup on PPC guns that had no lockup at all on the ejector rod.

put in a tight locking bolt and line up yoke.