For years, we have used size 14 ink sacs for Sheaffer Snorkels, size 15 for slender Sheaffer Touchdowns, and size 17 or 17 ½ for first-year fat Touchdowns, Touchdown Tuckaways, and PFMs. Recently, we have heard from a number of collectors confused by recommendations that other sizes be used, as well as special thin-walled sacs. In fact, there is no good reason to deviate from the standard sizes originally recommended by Sheaffer and their sac suppliers, the White Rubber Company. These size recommendations may be examined at the Pen Sac Company's website.
As for the thin-walled sacs, they would seem to be a solution to an imaginary problem. According to their proponents, thin-wall sacs are better for pneumatic filling systems, which allegedly have difficulty completely flattening the more resilient standard sacs. Yet we have never seen the slightest evidence of this, including when observing the operation of transparent demonstrators. A further indication of the ample ability of a pneumatic filler to empty a standard sac may be seen by filling a Snorkel with water (or ink) and then depressing the plunger. In this case the resilience of the sac accounts for but a minor part of its resistance to compression, the greater part being the resistance of the liquid inside to being forced out through the narrow passageway of the Snorkel tube. Nonetheless, even here the pneumatic filler quite nicely flattens the sac -- as one can tell by extending the plunger and depressing it once again to see if any more liquid is expelled.
Thin-wall sacs may also cause problems. They are more prone to aging, wear, and puncture, of course, but they also may reduce filling efficiency since their lesser resilience will slow down reinflation after flattening pressure is released.