THORIUM Carboy Cap 3, 5 & 6 Gallon Blow Off Hood with Caps Orange Silicone (4 Pack) Review (2026)

Who benefits from a four pack

THORIUM's carboy cap is a stretch on silicone hood designed to fit the mouth of 3, 5 and 6 gallon glass or plastic carboys without needing a separate drilled stopper sized to each carboy neck. Buying four at once suits a brewer who ferments in multiple carboys during the same season, whether that means running several batches in parallel or simply wanting a spare on hand when one cap is busy on a blow off tube setup during a vigorous fermentation. A brewer with only one carboy and no plans to expand will likely find four more than needed, though the cap works equally well as an everyday airlock holder even outside of blow off situations.

How the two port design works

Each cap has two molded ports on top, which is enough to run a blow off tube through one port into a jug of sanitized water while the second port holds a thermometer, a racking cane, or a second airlock, so brewers are not forced to choose between temperature monitoring and blow off protection during an active fermentation. The stretch fit silicone is meant to grip the outside of a standard 3, 5 or 6 gallon carboy neck rather than plug into the opening the way a rubber stopper does, which makes it faster to put on and take off between uses. This style is popular for the first few days of a vigorous fermentation when a batch is prone to foaming up and pushing krausen out through a standard airlock.

Fitting and using the cap day to day

Getting a secure fit works best when the carboy neck is clean and completely dry, since any residual water, sanitizer or oil on the glass or plastic can let the silicone slide rather than grip, which is the most common complaint with stretch fit caps of this style. Once fitted, a length of vinyl or silicone tubing runs from one port down into a sanitized jug or bucket of water, forming a blow off setup that lets escaping krausen and gas vent safely away from the fermentation area instead of building pressure under a standard airlock. After the vigorous first days of fermentation pass, many brewers swap the blow off tube out for a normal airlock in the same port and leave the cap in place for the rest of primary fermentation.

How it compares with other blow off options

Compared with a dedicated blow off tube kit, such as a length of one inch tubing paired with a drilled stopper, the THORIUM cap skips the stopper sizing step entirely since one cap works across three common carboy sizes, which is convenient for a brewery with mixed carboy sizes on the shelf. Two and three pack versions of the same cap are available for brewers who do not need four at once, and cost proportionally less, so this four pack is really aimed at brewers who know they will use every cap rather than someone testing the style for the first time. A plastic fermenter with a built in spigot and airlock port, such as a Speidel style vessel, sidesteps the fitting question altogether but costs considerably more than a set of caps for an existing carboy.

Bottom line for carboy fermenters

For a home brewery running multiple 3, 5 or 6 gallon carboys through vigorous fermentations, this four pack earns its our score of 8.6 out of 10 by giving each carboy a fast fitting blow off hood with room for a thermometer or airlock alongside the tube. The main thing to manage is making sure the carboy neck is clean and dry before fitting the cap, since a slippery neck is the one situation where this style underperforms a drilled stopper. Brewers who only run one carboy, or who prefer a stopper that seals into the neck rather than stretching over it, may be better served by a smaller pack size or a traditional drilled bung and blow off tube.

Frequently asked questions