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Many bulkhead fittings use a reverse, or left-hand, thread on the large retaining nut. This is intentional. It helps keep normal plumbing work from loosening the nut that clamps the fitting to the tank.
The important detail is that the whole fitting is not necessarily reverse threaded. On many designs, the large outside body thread and locknut are left-hand threaded, while the smaller pipe connection remains a normal right-hand pipe thread. Manufacturers including Hayward and several tank-fitting suppliers use this arrangement.
Why Bulkhead Fittings Use Reverse Threads
A bulkhead fitting has two different jobs:
- Clamp securely through the wall of a tank.
- Provide a connection for a pipe, valve, hose adapter, or other plumbing.
Those jobs often involve two separate sets of threads.
The large external threads hold the bulkhead locknut. The smaller plumbing threads accept the pipe or adapter.
If both threaded connections worked in the same direction, torque from installing or removing plumbing could encourage the bulkhead body or retaining nut to turn. A left-hand locknut thread helps separate those two actions.
In simple terms, turning a normal pipe fitting into the bulkhead is less likely to undo the nut holding the bulkhead onto the tank.
This matters because the bulkhead seal depends on keeping steady pressure on its gasket.
Which Part Is Actually Reverse Threaded?
Usually, it is the large retaining nut, not the water connection.
A typical threaded bulkhead may look like this:
| Part | Common thread direction | Purpose |
|---|---|---|
| Large outside body thread | Left-hand | Holds the bulkhead nut |
| Bulkhead locknut | Left-hand | Clamps fitting to tank |
| Female pipe connection | Right-hand | Accepts normal plumbing |
| Male pipe connection, if provided | Right-hand | Connects to normal plumbing |
For example, U.S. Plastic lists bulkhead fittings with left-hand external body and nut threads while the internal connection is standard female pipe thread.
This means you normally do not need special reverse-threaded pipe or valves. The reverse thread is part of the bulkhead's clamping system.
What Does Left-Hand Thread Mean?
Most screws and plumbing threads are right-hand threaded.
With a normal right-hand thread:
- Clockwise tightens.
- Counterclockwise loosens.
A left-hand thread works in the opposite direction:
- Counterclockwise tightens.
- Clockwise loosens.
That can feel wrong the first time you install a bulkhead.
Some manufacturers mold an arrow onto the nut showing the tightening direction. Hayward specifically tells installers to follow the direction arrow because its bulkhead nut-to-body connection uses left-hand threads.
Do not force a nut that seems unwilling to start. First check whether it has left-hand threads.
Why Keeping the Bulkhead Nut Tight Matters
The locknut itself normally does not create the watertight seal.
Its job is to pull the fitting flange firmly against the gasket and tank wall.
The basic arrangement is usually:
bulkhead flange → gasket → tank wall → retaining nut
As the nut tightens, it compresses the gasket against the tank.
That gasket is what keeps water from passing around the hole.
If the bulkhead body rotates while you install another fitting, the gasket can move, wrinkle, or creep away from its correct position. Poly Processing specifically warns installers not to let the bulkhead body slip or spin while tightening either the retaining nut or adjoining piping.
That is especially important with thin polyethylene rain tanks, barrels, and cisterns.
Reverse Threads Do Not Replace Proper Installation
A left-hand locknut helps keep the assembly secure, but it cannot correct a poorly installed bulkhead.
Several other things still have to be right.
The hole must fit the bulkhead
A hole that is too small can stress the fitting or tank wall.
A hole that is too large may leave too little surface for the gasket to seal reliably.
Do not choose a hole saw simply from the nominal pipe size. A 1-inch bulkhead, for example, needs a hole much larger than 1 inch because the threaded body must pass through it.
Hole requirements vary by fitting design. Use the manufacturer's dimensions for the exact bulkhead.
The gasket must sit flat
The sealing surface should be smooth and free of:
- Plastic shavings
- Deep scratches
- Raised molding seams
- Dirt
- Warped areas
The gasket should lie flat without being twisted or pushed partly through the hole.
The bulkhead should not spin
Hold the body while tightening the retaining nut.
You should also support the body when tightening threaded valves, nipples, or adapters afterward.
Consider practical steps for tightening bulkhead fitting to compare the choice within the overall harvesting system.
Otherwise, the torque can transfer through the bulkhead and disturb the seal.
Do not simply tighten harder if it leaks
Bulkhead fittings depend on controlled gasket compression.
Overtightening a plastic bulkhead can distort the gasket, damage threads, deform the tank wall, or crack the fitting. U.S. Plastic warns that excessive tightening can distort the gasket and cause leaks rather than cure them.
If a new fitting leaks, inspect the hole, gasket position, sealing surface, and fitting alignment before applying much more force.
Not Every Bulkhead Fitting Is Reverse Threaded
Left-hand locknuts are common, but they are not universal.
Different manufacturers use different designs. A bulkhead may have:
- Left-hand retaining threads
- Right-hand retaining threads
- Buttress-style threads
- Straight plastic threads
- Threaded pipe connections
- Slip/socket connections
- One gasket
- More than one sealing or friction washer
Even fittings that look nearly identical may work differently.
One Hayward installation manual notes that its fitting uses a left-hand body/nut thread, while other installation guidance recognizes that thread direction can depend on the particular bulkhead.
If you are unsure, look for a tightening arrow or check the manufacturer's instructions before putting a wrench on it.
Do Not Confuse Bulkhead Threads With Pipe Threads
This is one of the easiest mistakes to make.
The large threads visible on the outside of a plastic bulkhead may look like plumbing threads. Usually they are not.
They may be designed only for the bulkhead retaining nut.
The actual plumbing connection may be inside that threaded barrel.
For example, you might have:
- A large left-hand external thread for the locknut
- A 1-inch female NPT connection through the center
NPT, or National Pipe Thread, is a common tapered pipe-thread system. The tapered threads become tighter as the male and female parts screw together.
Do not try to screw a pipe fitting onto the large bulkhead-body threads unless the manufacturer specifically designed them as a plumbing connection.
Why This Matters on Rainwater Tanks
The reverse-thread design becomes useful when you start adding plumbing outside a rain tank.
A typical outlet might have:
tank → bulkhead → nipple or adapter → ball valve → hose or irrigation line
Installing the ball valve or adapter puts twisting force into the system.
If that force rotates the bulkhead, the gasket seal can be disturbed.
So when working on an existing rainwater tank:
- Support the bulkhead body when possible.
- Avoid using the connected pipe as a lever.
- Do not hang heavy unsupported valves or pumps from a plastic bulkhead.
- Check the gasket area after changing attached plumbing.
- Test the connection for leaks before relying on the tank.
This is particularly useful with rain barrels and thin-wall tanks, where the wall itself provides less support than a thick rigid vessel.
What If Your Bulkhead Nut Will Not Loosen?
If the nut seems to become tighter when you turn it counterclockwise, stop and check the thread direction.
On a left-hand bulkhead nut, clockwise may be the loosening direction.
Before applying more force:
- Empty the tank below the fitting.
- Check the nut for an arrow or molded markings.
- Confirm the fitting manufacturer's instructions if available.
- Hold the bulkhead body so it cannot rotate.
- Turn the retaining nut in the correct loosening direction.
Be especially careful with old plastic fittings. Sun exposure and age can make some plastics brittle, and excessive wrench force can crack the fitting or tank wall.
Frequently Asked Questions
Are all bulkhead fittings reverse threaded?
No. Many bulkhead fittings use left-hand threads for the retaining nut, but some use normal right-hand threads. Check the fitting markings or manufacturer's instructions before tightening or removing it.
Which direction tightens a reverse-threaded bulkhead nut?
A typical left-hand nut tightens counterclockwise and loosens clockwise. However, use the direction arrow or instructions supplied with your fitting when available.
Is the pipe connection also reverse threaded?
Usually not. Many bulkheads have a left-hand external thread for the retaining nut but a normal right-hand NPT or other standard plumbing connection for the pipe.
Why does my bulkhead have two different kinds of threads?
The threads do different jobs. The large body threads hold the locknut against the tank. The smaller plumbing threads connect the bulkhead to pipes, valves, or adapters.
Should I use thread sealant on the bulkhead locknut?
Do not assume the locknut threads need pipe-thread sealant. They normally provide clamping force rather than the water seal. Manufacturer instructions vary, and some specifically prohibit lubricants on the body-to-nut threads. The gasket provides the main tank-wall seal. Follow the instructions for the particular fitting.
Can overtightening a bulkhead fitting make it leak?
Yes. Excessive force can distort the gasket, deform a plastic tank wall, damage threads, or crack the fitting. If a bulkhead leaks, check the gasket, hole, alignment, and sealing surface instead of automatically tightening harder.
Does the gasket go inside or outside the rainwater tank?
On many common single-gasket bulkheads, the gasket sits against the flange side of the fitting, which is often installed inside the tank. Designs vary, however, so use the gasket arrangement specified by the fitting manufacturer rather than assuming every bulkhead is assembled the same way.
