Like many of our other systems of measurement, American pocket watch movement and case sizes can be traced back to an English system, one known as the Lancashire gage. In this system, the size is based upon the diameter of the watch plate to which the dial is fastened, known as the pillar plate. 0-size has a pillar plate diameter of 1-5/30" (yeah, right!). Nominal sizes then increment by 1/30" for each count. Although there are exceptions, most American pocket watches, and Swiss pocket watches made for the North American market, are sized to even numbers with the most common being 0, 6, 12, 16 and 18. These account for the vast majority of American-made watches. A 6-size watch has a pillar plate diameter of 1-5/30" plus 6/30", or 1-11/30". The diameter of a 16-size movement's pillar plate is 1-21/30", and an 18-size watch is one whose pillar plate diameter is 1-23/30" ( 1-5/30" plus 16/30" and 18/30" respectively). For sizes below 0-size (written x/0), the 1/30” is subtracted from the 1-5/30. Thus a 6/0-size watch has a pillar plate diameter of 29/30”. This is right up there with 12 inches to a foot, 16 ounces to a pound and 32 ounces to a quart. The same people have brought us the term hundredweight, which isn’t a hundred of anything but it is equal to 8 stone. Since a stone is equal to 14 pounds, a hundredweight is equal to 112 pounds.
The fact that even sizes are almost always used, alleviating the necessity of making very fine distinctions, makes it easy to express the watch size with a fairly close approximation. This doesn't always work, but it should get you close. First, measure the diameter of the dial to the nearest 1/32 of an inch, then subtract 1/32" and then subtract 1-5/32". For example, the dial diameter of a 16-size watch is 1-22/32". Subtracting 1/32", we get 1-21/32". Subtracting 1-5/32" leaves 16/32", Hence, its a 16-size watch. Remember to be careful of the hands when measuring the dial diameter. Also see What Size Is My Watch? on Barry Goldberg’s website. In lieu of calculating the size from a dimension, you can print out this scan of a Movement Scale and use it. It should get you close enough. Well ..... you'll probably have to resize your print (see below).
In order to measure the dial diameter, and only the dial diameter, don’t include any part of the case. The bezel, the metal ring that holds the crystal (glass), will have to be removed or opened. In the manner similar to the backs of watches, the bezel either hinges, snaps or unscrews open on an open-face watch and snaps off on a hunting-case watch. Refer to the How To Open A Pocket Watch Case Encyclopedia article.
To resize the print of the Movement Scale:
1. Save the picture to file.
2. Using Microsoft Photo Editor, open the file.
3. With the picture open in Photo Editor, click on File on the upper toolbar, then, in the drop-down menu that opens, click on Print and when the Print box opens, click on OK to print the picture.
4. Measure the length of 100mm on the printed picture in actual millimeters, using a suitable scale (ruler).
5. Divide: 100mm/Measured Length
6. Multiply by 100 to get percent.
7. Go to Print again, only this time, enter your calculated percentage in the Width: % field in the Size portion of the Print box.
8. Click on OK to print the picture. Check 100mm in the printed picture against a suitable scale (ruler). Adjust your calculated percentage and repeat steps 4-8 if needed.
One should keep in mind that a considerable number of watch movements are nominally sized and may only go in cases designed for those specific movements. This is especially true of a number of American-made watches built after WWI which were specifically sized to only mount in proprietary cases furnished by the movement manufacturers when supplying complete watches.
Categories: Category American watches