THORIUM Carboy Cap 3, 5 & 6 Gallon Blow Off Hood with Caps Orange Silicone (2 Pack) Review (2026)

Who this carboy cap suits

This THORIUM carboy cap is a flexible orange silicone cover sized to fit 3, 5 and 6 gallon carboy necks, sold here as a 2 pack for brewers who run more than one fermenter and want a matching cap on each. Unlike a rigid drilled stopper, the silicone material stretches to seat over the carboy opening rather than requiring an exact bore match, which gives it some tolerance across the handful of carboy neck sizes it is rated for. It suits brewers who want a single cap that can serve either as an airtight closure or as a mount point for a blow off tube, airlock, siphon or racking rod, depending on which of its two spouts is in use.

Design and how it seals

The cap has two openings described as breathable thermometer style spouts, which are convenient points to insert or attach tubing, an airlock, a siphon or a racking rod without needing to remove the whole cap each time equipment changes. Because the body is silicone rather than hard plastic, it flexes to grip the carboy neck and create a seal without needing precise threading the way a screw top lid does, which can make it quicker to put on and take off during a busy brew day. Reviewers note it gives a nice airtight seal with a place to fit an airlock, matching the manufacturer's description of it as a two spout blow off hood.

A known handling quirk

If the cap gets wet, whether from condensation, a spill, or blow off foam, it can lose enough grip to work partially loose from the carboy neck rather than staying firmly seated. Brewers who have run into this often recommend using a zip tie around the base of the cap as a simple backup to keep it secured, especially during the most active days of fermentation when foam and moisture are most likely to build up around the neck. This is a real tradeoff of a stretchy silicone seal compared with a rigid screw top lid, which does not depend on friction alone to stay in place.

How it compares with other carboy closures

Against a rigid one piece lid with an o-ring gasket, like the kind used on wide mouth plastic carboys, this silicone cap is more universal across the 3, 5 and 6 gallon neck sizes it covers, but it relies on flexible friction fit rather than a compressed gasket, which is part of why it can work loose when wet. Against stainless steel blow off tubes that insert into a drilled stopper, this silicone cap's two built in spouts remove the need for a separate stopper altogether, letting tubing or an airlock insert directly into the cap itself. The tradeoff is the seal depends on the cap's own grip on the neck rather than a stopper wedged into a fixed hole.

Verdict and who should pick something else

Choose this 2 pack if you run multiple 3, 5 or 6 gallon carboys and want a flexible, easy to swap cap that doubles as a mount for an airlock or blow off tube without needing a separate drilled stopper. Add a zip tie as routine insurance if your fermentation tends to get foamy or your cellar runs humid, since a wet cap is the main situation where it can work loose. Brewers who want a seal that does not depend on friction at all, or who have carboys outside the 3 to 6 gallon range this cap is rated for, should look at a rigid gasketed lid or a properly sized drilled stopper instead.

Frequently asked questions