Crystal replacement is necessary for a number of pocket watches which either have a broken or missing crystal, or which (in the ancient past) have been fitted with an early plastic crystal. The early plastic crystals deteriorate, discoloring and releasing gases that corrode the hands and, if left in place, will damage other steel parts.
Crystal SizeCrystal sizes are independent of movement sizes, except that cases of a given movement size will require crystals within a certain range of diameters. Always keep in mind that all cases made for the same size movement do not use the same crystal size. Also, the ranges of crystal diameters that might be needed for successive movement sizes will overlap.
A caliper, reading in thousandths (1/1000's) of an inch, or tenths (1/10's) of a millimeter, is needed to measure a bezel to determine the correctly-sized crystal. Bezels have slightly larger diameters inside the "bottom" of their groove, than inside their upper rim. To find the correct crystal size, one needs to measure that larger inside diameter. This is because the correctly-sized crystal is held in place by the upper rim, such that cement is not needed for a correctly-sized crystal. It simply 'snaps' into place.
Then, depending upon whether the measurement is in inches or millimeters, use the appropriate chart to find the right crystal size. Or, crystal suppliers ought to be able to furnish a correctly-sized crystal directly from your measurement. Depending upon the cost of the crystals, it might be advisable to get one or two sizes immediately larger and smaller than the one indicated by the measurement, especially if expect to continue collecting the same size watches.
Crystal ChartsThe Crown crystal size chart, and its explanation, may be helpful. Also, thanks to Tom McIntyre's post on the Message Board, we have access to a Crystal Size to Decimal Inch Conversion Chart and thanks to mrbill we can view this Conversion Table.
You can avoid all of the charts and figuring sizes out by simply measuring the inside diameter of the bezel and contacting a supplier with the measurement. Anybody who sells crystals should be able to work from that.
Also see Dave Coatsworth's post on the subject.
Obtaining CrystalsWatch Materials are available from:
Brian Cavanaugh, email@example.com
Bryan Eyring, firstname.lastname@example.org
Uncle Larry's Watch Shop
Also, sderek has reported a crystal replacement service in this Message Board thread.
Mounting the CrystalWhen a glass crystal is sized correctly to the metal bezel that holds it, it is an interference fit and will not need cement to hold it in place. To fit a crystal into the bezel, set the bezel on a flat surface, facing upward, and place the crystal on top. If the crystal is slightly too small, it will drop right into place and be loose within the bezel. A correctly-sized crystal will "almost" fit within the inner rim of the bezel, appearing to extend beyond the rim of the bezel by a tiny amount. If this is a crystal that had previously resided in the bezel, it is correctly sized. If it is a new crystal, be careful that it is not too large for the bezel. The correctly-sized crystal can be "snapped" into place by placing your thumbs on opposite sides of the bezel, overlapping the crystal. As you press down firmly (but not excessively) and roll your thumbs together, the correctly-sized crystal will seat with an audible "snapping" sound.
Cementing the Crystal in PlaceIf the crystal is slightly too small, and a correctly-sized crystal is not immediately available, it can be held in place by the use of crystal cement. This is viewed as a less-than-ideal situation, but nevertheless, many of us go this route. Crystal cement, available from watch material supply houses - see above, is specially formulated to prevent the release of gases that can leave a residue on the crystal and also to stay slightly pliable. If you don't see crystal cement listed on the above websites, contact them and ask about it. Crystal cement is somewhat removable, more so than many household cements (which shouldn't be used).
Crystal Replacement ServiceHaving said all of above, an easy way out is to send the bezel or whole watch to a crystal replacement service. Good reports have been reported about the service mentioned in the thread entitled "Watch Crystal needed".