Watch Crystal Replacement 
One of the best ways to make your vintage watch look great, (and to add value) is to replace a scratched, cracked, or chipped crystal.  Many watchmakers can do this for very little cost - usually about $30 or $40 for a plastic crystal.  Be ready to pay the watchmaker a little more for mineral glass or those rare faceted or beveled varieties...
If you are a watch repair hobbyist you may want to try to tackle this job yourself.  In most cases it is surprisingly easy to do if you can locate a new replacement crystal.  (but can sometimes be a bit tricky in some situations without the right tools.) - 
Let's get started:

1) Open the case.  In most old "tank" watches this is as simple as finding the raised "prying lip"  or groove in the case and prying open the case as seen below..
2)  Pop out the crystal through the front of the case using gentle pressure.  (HINT;  Use this opportunity to clean the case.  Use a piece of pegwood or a toothpick to remove the old glue from the groove where where the old crystal sat)
Many times the old crystal can be cleaned up and polished using a light polishing compound.

Place a bead of Crystal Cement around the inside lip / groove where the crystal goes, and snap in your new crystal in place.

*****HINT:  Be sure to allow the glue to dry completely before reassembling the watch as it is possible to get glue on the dial****
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