Fountain Pen Sac Size Guide
(This page revised December 18, 2017)
The charts on this page are a guide to choosing appropriate sac sizes for various sac-filling pens. The sizes listed here are not the sizes the original manufacturers used. Pen manufacturers chose the largest sacs they could cram into their pens in order to yield the greatest possible ink capacity, but there are some drawbacks to that method:
- A larger sac is more difficult to insert into the barrel. Even if coated with talcum powder, it can bind against the barrel wall or the pressure bar, and it can twist and crinkle as the repairer tries to stuff it all the way in.
- Because modern repairers are dealing with used pens, pressure bars can be distorted and there can be buildup on the surface of the pressure bar or barrel wall. These conditions make it even more difficult to fit a sac of the original size.
- Because more of its surface is in contact with the barrel wall, a larger sac absorbs heat from the user’s hand more readily than will a smaller sac. This causes any air in the sac to expand, increasing the pen’s flow unpredictably and possibly causing the pen to blot, especially when it is nearly empty.
For these reasons, the charts recommend sacs that are usually one or two sizes smaller than the sizes originally used; but in some cases, the difference can be as great as four sizes. In most cases, the smaller sac size vitiates the need for a necked sac; unless otherwise specified, all sacs listed here are straight sacs. For pens not listed, choose a sac that drops freely into the barrel with the pressure bar in place.
If you do not see your pen listed in the chart, you can always measure to establish sac size. Measure the diameter of the section nipple where the sac attaches with callipers. Next conver the decimal size to the nearest 64th. fraction. For example 0.2344 is 15/64ths. this would be the largest sac size possible for your pen. You then must take into consideration the available clearence in the pen barrel. Replacement button and J bars can easily reduce the size of the sac by two sizes.
Despite the fact that latex sacs do not last forever and will need to be replaced, I still recommend them as the best available alternative. Do not use PVC sacs, which will bond themselves chemically to the barrel wall of many pens. I also dislike silicone sacs; they will not leak ink, but they are gas permeable and will allow air to pass into them such that the pen will leak ink through the nib and feed if it is stored horizontally or nib downward.
Note: If you are using a modern two-piece replacement pressure bar (illustrated here), choose a sac one or two sizes smaller than indicated in the chart.
This chart and the information within are © RichardsPens.com. Used with permission.
- A correct Pli-Glass sac is not available for the Demi. A Nº 14 vinyl (often sold as “silicone”) will fit when cut to 2" long; there are also some vinyl sacs available that resemble the original “51” sac but are too small for a full-size “51” — these sacs will work well in a Demi.
- The correct size sac for the Touchdown TM, as noted in the table, is 15×21/8 Necked. At the time of this writing, this sac was not available from any known source. Substitute a Nº 15 straight sac cut to a length of 23/8".
The information in this article is as accurate as possible, but you should not take it as absolutely authoritative or complete. If you have additions or corrections to this page, please consider sharing them with us to improve the accuracy of our information.