I assume you are talking about what you consider excess up and down movement in the thumbpiece when it is at rest. As you know, the thumbpiece is attached to the bolt, which rides fore and aft in a frame slot. Because of the design of the components, some gauge is necessary (normal) in this assembly, and should not affect normal function.

You didn't say exactly how the "failure to open" malfunction is occurring. If you are pushing (able to push) the thumbpiece fully forward, and the cylinder will not come open, I would suspect that either the extractor rod is loose, or the extractor rod end that interfaces with the front locking bolt is not fit correctly. When correctly fitted, the center pin should come all the way to the end of the opening in the extractor rod, or protrude slightly past the end of the rod (.005-.010") when the thumbpiece is fully forward.

Another possibility would be end shake on the cylinder, which would move the cylinder slightly forward, causing an improper fit at the interface of the extractor rod end and the front locking bolt.

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Ret. LE, FA Instr, S&W Armorer