This is, in broad terms, where the ink lives. In the earliest pens, this was simply the interior of the barrel, and in some pens of more recent make the back end of this space contains a mechanism. In modern pens, it is frequently a detachible plastic ampoule, some of which have a mechanism built into them. Capillary fillers use sheets of more or less absorbent material as a reservoir, with the ink contained in the thin spaces a rolled or stacked flat material provides. In the majority of pens, taking vintage and modern together, the reservoir is a flexible sac filled by compression by some means.
There are three main materials sacs are made from. Most modern pens using a sac will have a clear one in a press-bar filler housing. The more traditional material, used in vintage pens of various mechanism and some modern converters, is rubber, essentially the same stuff as would be found in a bicycle inner-tube. A modern alternative to the rubber sac is one made of silicon; this has the advantage of being rather inert chemically, and thus unlikely to cause ambering, but is less resilient than rubber and thus harder to attach and to fill. Generally, where ambering isn’t an issue, rubber is the preferred material in replacement sacs, while where ambering may occur there is a bit of a debate whether it is worth the hassle of installing silicon. Personally, I incline somewhat towards switching to silicon for preservation’s sake, although if the pen is maintained properly the hazard of ambering is minimized.
One of the more obscure notions about rubber (and silicon) pen sacs is its number, as in, “This pen uses a #14 sac.” The number of a pen sac is equal to the numerator when the outside diameter of the sac is expressed in sixty-fourths of an inch. A #18 sac is 18/64″ across. This sort of thing, by the way, is why most countries have switched to the metric system.
To measure a pen that you don’t know the correct size of sac for, there are two things to take into account. First, the inner diameter of the barrel (including any space occupied by the mechanism) should be bigger than the outer diameter of the sac; this allows for a little bit of an insulating buffer of air, reducing chances of a leak. Second, the outer diameter of the nipple be slightly bigger than the inner diameter of the sac, as the sac should stretch to fit rather than simply slide over the nipple. Usually, the second consideration takes care of the first, but one is still left with a problem– the measurement for either doesn’t have direct bearing on the sac number. As a rule of thumb, measure the nipple, and apply a sac with a number equal to or just less than that measurement. Since the sac number is measuring its exterior, you’re adding the thickness of the material to the measurement of the nipple, and that should provide a good fit.
Not all of these will be found in all pens, and there are a few pens so specific in their parts I likely won’t remember to include them here.
Blind Cap: Removable tail potion of barrel which covers the actuating part in button-fillers and some piston-fillers, and acts as a handle in vacuum and pneumatic pens. In other piston fillers, this part remains more or less in the same place while acting as the knob for the filler, and is a false blind cap. A false blind cap may also be a mere decoration in a pen with some other filling mechanism.
Breather tube: Allows air out of the back of the reservoir, mainly so that multiple workings of the mechanism will result in a more complete fill. Found in accordion, bulb, and press-bar fillers, and in very rare cases in a lever filler.
Capsule: Sealed container for the reservoir material of a capillary filler.
Diaphragm: Flexible rubber component in a Vacumatic pen.
Driver: The “nut” in a piston filler that transfers the rotary motion from the knob onto the piston shaft’s linear action.
Filler Tube: The moving part in a Sheaffer pneumatic filler, attached to the inner end of the blind cap.
J-Bar: A specialized pressure bar, specific to lever fillers.
Nipple: The portion on the back of a section to which either a sac or a cartridge is applied, depending upon the type of filler.
Pellet: A small bead in the mobile end of a diaphragm which provides the necessary friction to hold it onto the shaft a Vacumatic filler, or a small bead inside a converter which helps prevent ink from becoming trapped by surface tension by mechanical disruption.
Pellet Cup: The attachment point for the mobile end of the diaphragm in a Vacumatic filler.
Piston Seal: In a piston filler, the gasket at the end of the piston shaft that holds ink in the chamber. In a vacuum filler, the valve at the interior end of the shaft which governs the filler’s action.
Pressure Bar: Just about anything that presses upon a sac.
Sac Protector: In Sheaffer pneumatic pens, a cage for the sac that prevents distention of the sac or other parts directly touching it. In press-bar pens, a similar enclosing shroud in which the pressure bar is mounted.
Spring: Part of the pressure bar in a button-filler (flat), or an alternative term for a j-bar (flat), or the means of returning a Vacumatic filler to its rest position (coil), or the means of keeping the sac/probe unit in a Sheaffer Snorkel pen fully extended (coil), or a variant on the pellet in a converter(coil). Very useful items, springs.