A plastic crystal that...August 3 2010 at 12:04 AM
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Response to A simple Water Resistant test...
is the right size for the bezel, has not chips in its rim, and is properly snapped into the bezel will generally give good water resistancy. As you noted, however, the large circumference of a watch's back cover can offer alot of opportunity for leakage if there is not gasket present there.
Your watch's stem tube probably has no O-rings along its length to prevent water from infiltrating between the stem and the stem tube. That prevention is being solely provided by a tiny tubular neoprene piece inside of the crown. When the stem is pushed completely in, that piece overrides the end of the stem tube a bit and prevents water infiltration. However, on most of the watches I've seen with "water resistant" crowns, that piece of neoprene inside the crown will still be overing some of the end of the stem tube even when the crown is pulled out to the time setting position. Thus, with these crowns the watch case is protected from water infiltration no matter what position the watch's crown is in. Of course, these watches are not intended for swimming or diving which can dramatically increase the "dynamic" water pressure exerted on their crown seals when the swimmers arms move rapidly about. They are certainly acceptable for a bath or showering, however.
Sadly, when one purchases a vintage "water resistant" wristwatch, he should, just to be on the side of safety, assume that most of the neoprene in its crown disintegrated / wore away a long time ago and that the watch is no longer water resistant. If one wants the watch to be RELIABLY water resistant again, then the back cover gastket and the crown should be replaced.
However, IF the back cover gasket is still intact, there was a method suggested here months ago that involved injecting a SMALL amount of liquid neoprene (a sealant available at hardware stores and Sears) into a vintage watch's crown and then pushing it back into the stem and letting the solvent in it evaporate (gentle heating of the case will speed this up, but, even so, it might still take a day or more for all of the solvent to leave the neoprene so it can become a hard rubbery material). This MIGHT be able to rebuild the neoprene piece in the crown and again restore the water resistancy of the watch case.