The rubber or silicone ring invariably wears out, cracks, splits or otherwise no longer seals properly.
Interestingly, I noticed that when browsing for replacements, there are a number of different pseudo-standard sizes involved.
Since they are flexible, one size too small may fit anyway, even if the gasket slightly buckles or curls, and sometimes the thickness varies enough (especially if too thick) that means you will need to adjust the wire band to accommodate the additional thickness (sometimes the lower band – the one on the jar itself, can be flipped upside down to give an additional 2mm of closure clearance for thicker 4mm gaskets – most gaskets are of the 2mm variety.)
Common Canning Wire Bail Jar Gasket Sizes with Inside and Outside Diameter measurements:
* Antique gaskets were usually only 1/4″ wide instead of the modern 1/2″ width.
For reference, Modern Standard Ball/Mason Screw-on Canning Lid sizes:
Standard Mouth Ball 2-1/2″ (on center at lid seal) – 2-3/8″ (60mm) ID 2-5/8″ (67mm) OD – at jar opening.
Wide Mouth Ball 3-1/8″ (on center at lid seal) – 3.0″ (76mm) ID 3-1/4″ (83mm) OD – at jar opening.
There are also any number of decorative model jars which were very tiny in comparison (1″ 25mm ID) which were not meant for actual canning use, but were often used for salt and pepper shakers or for gifting jam samples. Naturally, getting replacement lids or gaskets for these is pretty much impossible other than finding something that would work at your local hardware store in the pipe and plumbing department.
If you Care a Little More, Things Happen. Bees can be dangerous. Always wear protective clothing when approaching or dealing with bees. Do not approach or handle bees without proper instruction and training.