Bad Cleanout Threads? Try This Drain Plug Fix
A floor drain cleanout should seal tightly.
But that gets hard when the threads are damaged.
This happens a lot in older drains.
The plug may start crooked.
It may spin without grabbing.
It may also refuse to thread back in.
In this case, the old plug is a 1.5-inch NPT PVC cleanout plug.
The outside thread size measures about 1.89 inches.
That lines up with 1.5-inch nominal NPT pipe thread sizing.
You can compare pipe thread sizes with this pipe thread dimension chart.
The problem is not only the size.
The plug style matters just as much.
A regular threaded cleanout plug needs good female threads.
If those threads are stripped, crossed, or worn, it may not seal.
That is when many homeowners look for a rubber expansion plug.
Question
What rubber plug should I buy for a floor drain cleanout with bad 1.5-inch NPT threads?
Answer
Look for an inside-of-pipe mechanical expansion cleanout plug sized for 1.5-inch pipe, not an end-of-pipe test plug.
This is the key detail.
An end-of-pipe plug seals at the open end of a pipe.
That style is often used for testing drain lines.
It may not work inside a cleanout fitting.
It also may not seat behind damaged threads.
Some manufacturers sell both styles.
For example, Oatey explains that some gripper plugs are end-of-pipe designs.
Oatey also notes that inside-of-pipe designs can sit deeper inside the pipe.
You can read more on the Oatey Gripper plug page.
What To Search For
Search terms can be tricky on Amazon or eBay.
Try these words instead:
- 1.5-inch inside pipe expansion plug
- 1-1/2 inch mechanical cleanout repair plug
- 1-1/2 inch rubber expansion cleanout plug
- cleanout plug for damaged threads
- flush mechanical cleanout repair plug
Avoid listings that only say “end-of-pipe plug.”
Also, avoid simple rubber sink stoppers.
Those are not cleanout repair plugs.
They may pop loose under drain pressure.
Check The Fit Before Buying
Measure the cleanout opening, not only the old plug.
The rubber section must expand inside the pipe or fitting.
Check the product’s minimum and maximum sealing range.
Do not rely on the title alone.
Many listings use loose words.
Some sellers mix test plugs and repair plugs together.
If the listing has no size chart, skip it.
If it does not say inside-of-pipe, be careful.
If this cleanout may hold sewer backup pressure, use extra care.
A temporary test plug may not be the best long-term fix.
A licensed plumber can also chase the threads.
They may install a new cleanout fitting if needed.
That can be the safer fix for badly damaged drains.
Bottom Line
Your 1.89-inch thread measurement points to 1.5-inch nominal NPT.
But you are not shopping for another threaded plug.
You need a mechanical expansion repair plug.
Choose one made to seal inside the cleanout opening.
Do not buy an end-of-pipe plug again.
That one solves a different problem.
I’m Chris Mayer, writing for Plumbing 101.
