Identify the part that is missing
A vessel opening may need a bung, while an already drilled stopper may need only an airlock. A damaged main access lid is a different replacement from a small threaded port cap. Start with the actual removed part and the vessel instructions before comparing package counts.
Check both interfaces
For a bung, check the tapered body against the vessel and its drilled hole against the accessory stem. Number 10 and Number 6.5 designate different stopper formats. Universal-style wording is not a verified fit for every glass or plastic neck.
Compare access and package contents
A cylindrical three-piece airlock separates into components; the S-shaped format has a different chamber layout. The FastRack offers also differ in quantity and supplied bung type. If your existing stoppers fit, buying a separate compatible replacement may be different from ordering a whole set.
Use exact replacement references
For Speidel, compare FE787, FE780 and FE781 by their separate closure and vent roles. For FerMonster, distinguish the solid lid from the opener, whose stated fit is narrower than the lid family. Neither a solid closure nor accessory ports establish a pressure-use permission.
Compare the job and the buying check
FastRack Twin-Bubble Airlocks with Universal-Style Bungs, Two Sets
Two S-shaped water airlocks and two tapered carboy bungs for adding a visible liquid-seal vent to a compatible vessel.
Check: Check both interfaces: the bung-to-vessel opening and the airlock-stem-to-bung hole.
FastRack Three-Piece Airlocks with Number 6.5 Stoppers, Three Sets
Three cylindrical airlocks with removable internal parts, supplied with three number 6.5 rubber stoppers.
Check: Match the stopper to the actual vessel neck and the airlock stem to its drilled opening.
Acellegic Number 10 Drilled Rubber Stoppers, Six Pack
Six tapered drilled rubber stoppers for matching a vessel opening to a separately chosen fermentation airlock.
Check: Check the taper against the vessel opening and the drilled hole against the airlock stem as separate measurements.
Fermentology Two-Port Silicone Carboy Caps, Four Pack
Four stretch-on silicone carboy caps, each with two accessory ports, removable plugs and parking posts for those plugs.
Check: Choose by the accessory connections you need and the actual glass neck, not just nominal vessel volume.
Further questions
Should I buy a new opener whenever I buy a replacement lid?
Only if the actual lid and opener fit information support that choice. The selected FerMonster opener states 6 and 7 gal fit, rather than every lid in the family.
Can a two-port silicone cap replace any threaded lid?
It is a stretch-on accessory for its stated glass-neck family, not a universal threaded-lid replacement.
What should I record before ordering a small port cap?
Keep the vessel model, removed part reference, thread or taper and seal arrangement. A rough outside diameter is not a complete identification.
