Plumbing 101: Valve stem keeps spinning.

Hi everyone,
So, what am I doing wrong here? I am replacing leaky valve stems in a tenants shower, and when they turn open or closed, they keep spinning. The old ones stop at full open or closed.
I believe that these are cartridge stems, but I am not 100% positive.
I have tried installing them half open, fully open, and fully closed, and still I can't make them stop spinning. The old ones I can install any fashion I want, and they will stop at the appropriate places. I also tried oiling the new ones, to reduce resitance that might be making the "closer" pull towards the handle, as oppsed to vice versa.
I have compared old with new and they are identical.
Thanks for any input on this, I know it's plumbing 101.
Ryan
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Also, another major problem is that when I turn the water all the way open, the valve will spin freely and not shut off until I push it in a little bit, at which point I can feel it "bite" and then it shuts off after a few more spins.
Could this be cause by a slightly too-short stem? Like I said, the old and new appear identical to the naked eye, but they could be 1/16" or 1/8" different in length, I suppose.
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The Chairman wrote:

Sounds like they are the wrong stems (thread size), or else the threads in the valve body are worn out.
Jim
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