As an affiliate, we may earn a commission from qualifying purchases. We get commissions for purchases made through links on this website from Amazon and other third parties.
The gasket on a bulkhead fitting usually goes between the bulkhead flange and the tank wall, on the side where the large fixed flange sits. In most rain barrels, tanks, and cisterns, that means the gasket is on the inside, or wet side, of the tank.
The gasket should not normally go under the tightening nut unless the fitting manufacturer specifically says to put it there.
How a Bulkhead Fitting Seals
A bulkhead fitting passes through a hole in the wall of a tank or barrel. It normally has four main parts:
- A fixed flange or wide head
- A rubber gasket
- A threaded body
- A large retaining nut
The gasket seals the tank wall. The threads do not make this seal.
When you tighten the nut, it pulls the flange toward the tank wall and compresses the gasket. That compression keeps water from leaking around the hole.
A typical arrangement looks like this:
Inside the tank:
Flange → Gasket → Tank wall → Nut
Outside the tank
This is the most common setup for rainwater tanks and barrels.
Why the Gasket Usually Goes on the Flange Side
The gasket needs a smooth, broad surface pressing against it.
The bulkhead flange provides that surface. When the retaining nut is tightened from the other side, the flange presses the gasket evenly against the tank wall.
Putting the gasket under the nut can cause problems because the nut may:
- Twist the gasket while tightening
- Push the gasket out of position
- Compress it unevenly
- Make the fitting harder to seal
The nut's main job is to clamp the assembly together. It is not normally the sealing surface.
Does the Gasket Have to Be Inside the Tank?
Not always.
The important rule is:
Put the gasket directly under the bulkhead flange, unless the fitting instructions specify another arrangement.
Many bulkheads are installed with the flange inside the tank and the nut outside. In that case, the gasket is inside.
Some installations place the flange outside and the nut inside. If the fitting is designed for that orientation, the gasket would normally stay with the flange and therefore be outside.
For rain barrels and storage tanks, placing the flange and gasket on the wet side is common because it keeps the sealing surfaces protected and leaves the nut accessible from outside.
What If the Bulkhead Fitting Has Two Gaskets?
Some bulkhead fittings are supplied with two rubber washers or gaskets.
Do not automatically assume one belongs on each side of the tank.
The correct arrangement depends on the fitting's design. Some manufacturers provide an extra gasket for specific installations, while others design the fitting to seal on both sides.
Follow the instructions supplied with the fitting when they differ from the standard flange-and-gasket arrangement.
Adding an extra gasket where one was not intended can sometimes make the fitting less stable or allow it to shift.
How to Install the Gasket Correctly
Before installing the bulkhead, make sure the area around the hole is smooth and clean.
1. Check the hole
The hole should match the size required for the bulkhead fitting.
A hole that is too large may leave too little tank wall for the gasket to seal against. A rough or uneven hole can also create a leak path.
Remove loose plastic burrs without thinning or damaging the wall.
2. Clean the sealing surface
Dirt, plastic shavings, algae, or grit between the gasket and tank can prevent a good seal.
Wipe both surfaces clean.
3. Put the gasket against the flange
Slide the gasket onto the bulkhead body until it rests flat against the back of the flange.
Make sure it is not folded, stretched, or twisted.
4. Insert the fitting through the tank wall
For the common arrangement, insert the bulkhead from inside the tank so the order becomes:
Flange → Gasket → Tank wall
The threaded portion will extend outside.
5. Install the retaining nut
Thread the nut onto the bulkhead from the outside.
Keep the fitting from rotating while tightening.
6. Tighten carefully
The gasket needs enough compression to seal, but excessive tightening can cause problems.
Overtightening may:
- Squeeze the gasket out of position
- Distort a thin plastic barrel
- Crack a brittle tank wall
- Damage the bulkhead threads
- Warp the fitting
Follow the fitting manufacturer's tightening instructions when available.
Should You Use Silicone Sealant on a Bulkhead Gasket?
A properly sized and installed bulkhead fitting generally relies on its gasket rather than sealant around the tank hole.
Consider practical steps for fiting a bulkhead to understand the connection between this choice and system operation.
Adding silicone does not fix problems such as:
- An oversized hole
- A damaged gasket
- A warped tank wall
- A cracked fitting
- Dirt under the gasket
- Incorrect gasket placement
Sealant can also make future removal more difficult.
If the manufacturer specifies a sealant or lubricant, use only a material compatible with the tank, gasket, fitting, and intended water use.
For systems supplying water that may eventually be treated for drinking, material compatibility needs extra care. Do not assume ordinary household sealants are suitable for potable-water contact.
Does Thread Tape Go on a Bulkhead Fitting?
Thread tape does not replace the bulkhead gasket.
The large retaining nut threads are generally there to clamp the fitting against the tank wall. The gasket makes the tank seal.
Thread tape or another approved thread sealant may be needed later where a pipe, valve, hose adapter, or other threaded component connects to the bulkhead.
That is a separate seal from the gasket around the tank wall.
Why a Bulkhead Fitting May Still Leak
If water appears around the fitting, check the installation before simply tightening it harder.
Common causes include:
The gasket is on the wrong side
If the gasket is under the retaining nut instead of under the flange, move it to the flange side unless the fitting instructions say otherwise.
The gasket is twisted
A folded or pinched gasket may leave a small path for water.
Remove the fitting and reinstall the gasket flat.
The fitting rotated while tightening
Rotation can drag the gasket out of position.
Hold the flange steady while tightening the nut.
The tank wall is curved
Bulkheads seal best against reasonably flat surfaces.
A strongly curved rain barrel may prevent the flange and gasket from making even contact. Use a fitting designed for the surface rather than forcing a standard rigid bulkhead into position.
The hole is rough or too large
Deep scratches, burrs, or an oversized opening can extend beyond the gasket's sealing area.
The fitting is overtightened
More torque does not always mean a better seal. A distorted gasket or tank wall may actually leak more.
The gasket is damaged
Old rubber can crack, harden, flatten, or tear.
Replace a damaged gasket rather than trying to compensate with extra tightening.
Bulkhead Fittings on Flexible Tanks
IBC totes and many rigid cisterns provide fairly firm mounting surfaces. Thin rain barrels and flexible tanks require more care.
If the wall bends significantly while the bulkhead nut is tightened, the gasket may not compress evenly.
Avoid using the plumbing connected to the bulkhead as a structural support. Heavy valves, long pipe runs, or pumps can place sideways force on the tank wall.
Support external plumbing separately when necessary.
Bulkhead Fittings Below the Waterline
A bulkhead installed near the bottom of a tank is exposed to water whenever the tank contains water.
Check the connection for leaks before putting the system into normal service. Filling the tank gradually makes it easier to catch a problem before the tank contains its full volume.
If the fitting is difficult to reach from inside once the tank is full, checking it early is especially useful.
For very large cisterns, do not enter the tank to repair a fitting unless the tank is specifically designed for safe entry and proper confined-space procedures are being followed. Use an accessible fitting arrangement or qualified help instead.
Frequently Asked Questions
Does the rubber gasket go inside or outside a rain barrel?
Usually inside. On a common rain-barrel bulkhead, the flange and gasket are inside the barrel, while the retaining nut is outside. Keep the gasket directly against the flange unless the fitting manufacturer specifies otherwise.
Does the gasket go between the nut and the tank?
Usually no. The gasket normally goes between the fixed flange and the tank wall. The retaining nut clamps the fitting together.
Can I put a gasket on both sides of the tank?
Only if the bulkhead fitting is designed to use two gaskets. Adding a second gasket to a fitting designed for one does not necessarily improve the seal.
Do bulkhead threads need Teflon tape?
The threads used by the retaining nut normally do not need thread tape because they do not make the tank seal. A threaded pipe or valve screwed into the bulkhead may require an appropriate thread sealant.
How tight should a bulkhead fitting be?
It should be tight enough to compress the gasket evenly without deforming the tank, gasket, or fitting. Exact tightening requirements vary by fitting, so follow the manufacturer's instructions when available.
Can I use silicone to stop a leaking bulkhead?
Silicone should not be the first fix. Check gasket position, hole size, surface condition, fitting alignment, and gasket damage first. Use sealant only when it is appropriate for the fitting and intended water use.
Why does my bulkhead leak after I tighten it?
The gasket may be twisted, the tank wall may be curved, the hole may be damaged, or the fitting may be overtightened. Remove the fitting and inspect the sealing surfaces rather than continuing to tighten it.
