Oxygen Sensor Bung Plug FAQ: Common Questions Answered

Quick Answer

An oxygen sensor bung plug seals an unused O2 sensor hole in your exhaust so you can run without a sensor in that spot. Match the thread size to the bung, install it with the right torque, and use a high-temperature anti-seize so it can be removed later. If it leaks, the usual causes are a mismatched thread, missing or damaged gasket, or insufficient torque.

If you have added an aftermarket exhaust, deleted a rear sensor, or swapped to a wideband setup, you may be left with an open O2 bung. Leaving it open creates a loud exhaust leak, throws off readings, and can trigger a check engine light. A bung plug is the simple fix, but the details matter: thread size, sealing method, and torque.

This FAQ answers the questions we hear most about oxygen sensor bung plugs, including what they do, how to choose the right one, how tight to tighten them, and how to stop leaks. The answers are practical and based on standard exhaust practices, not marketing claims.

What is an oxygen sensor bung plug and what does it do?

A bung plug is a threaded plug that screws into an oxygen sensor bung, the welded-on fitting in your exhaust pipe that accepts an O2 sensor. Its purpose is to seal that hole when no sensor is installed. That keeps exhaust gases inside the pipe, prevents a loud leak, and protects the bung threads from corrosion and debris.

You might use one when you install a test pipe, run a standalone ECU with a different sensor location, or remove a rear O2 sensor after a tune. The plug itself is simple, but it must match the thread and seal properly to do its job.

• Common bung thread sizes include M18 x 1.5 and M12 x 1.25, but always confirm your bung size before ordering.
• A plug with a copper or crush washer seals better than a bare metal plug.
• If you plan to reinstall a sensor later, apply anti-seize to the plug threads, not the sensor threads, unless the sensor manufacturer says otherwise.

How do I choose the right size and type of bung plug?

Thread size is the first thing to confirm. Most oxygen sensors use M18 x 1.5 threads, but some newer vehicles and aftermarket bungs use M12 x 1.25 or other sizes. If you are unsure, measure the inside diameter of the bung or check the thread pitch with a gauge. Ordering the wrong size is the most common reason a plug will not seal.

Material and sealing style matter too. Stainless steel plugs resist corrosion, which is important in exhaust environments. A plug with a built-in crush washer or a separate copper gasket creates a better seal than a bare plug. Some plugs also have a hex head for easy installation; others use an Allen key. Choose based on the access you have around the bung.

• If the bung is in a tight spot, a low-profile Allen plug may fit where a hex-head plug will not.
• Avoid mixing thread types; never force a plug into a bung.
• When replacing a sensor with a plug, keep the old sensor as a spare if it still works.

What is the correct torque spec for a bung plug?

There is no universal torque spec because it depends on thread size, plug material, and whether you use a gasket. A safe starting point for an M18 x 1.5 steel plug with a copper gasket is 18–22 ft-lbs (25–30 Nm). For M12 x 1.25, use 12–15 ft-lbs (16–20 Nm). These are general ranges, not a substitute for the plug manufacturer's instructions.

Over-tightening can strip the bung threads or crush the gasket too much, causing a leak. Under-tightening leaves the plug loose and allows exhaust to escape. Use a torque wrench if you have one. If you are reusing a bung that has seen many heat cycles, clean the threads with a wire brush before installing the plug.

• Always apply anti-seize to the plug threads to prevent galling, especially with stainless steel.
• Tighten in a smooth, steady motion; do not jerk the wrench.
• After the first heat cycle, recheck torque once the exhaust is cool.

Why is my exhaust bung plug leaking and how do I fix it?

A leak usually comes from one of three things: a thread mismatch, a missing or damaged gasket, or insufficient torque. If the plug threads in easily but still leaks, the threads may be the wrong pitch. If it threads in but weeps, the gasket may be missing or crushed. If it is tight but still leaks, the bung itself may be cracked or the threads may be damaged.

To fix a leak, remove the plug and inspect the threads and sealing surface. Clean the bung threads with a wire brush or thread chaser. Replace the gasket if it is flattened or missing. Reinstall with anti-seize and torque to the correct spec. If the bung threads are damaged, you may need to replace the bung or use a thread repair insert.

• A leak can sound like a ticking noise that changes with engine speed.
• Do not use silicone sealant; it will not survive exhaust temperatures.
• If the bung is cracked, welding a new bung is the permanent fix.

Can I use a bung plug with a MagnaFlow exhaust or catalytic converter?

Yes, as long as the thread size matches. MagnaFlow performance exhaust systems and catalytic converters often come with bungs for oxygen sensors. If you are not using a sensor in that location, a plug is the right way to seal it. For example, the MagnaFlow 1980-1981 Chevrolet Corvette 5.7L Street Series Cat-Back Performance Exhaust System 16889 uses mandrel-bent 2.5-inch piping and may have bungs for sensors; check your specific system for bung size and location.

MagnaFlow also offers universal catalytic converters like the MagnaFlow OEM Grade Federal/EPA Compliant Universal Catalytic Converter 51506, which may require you to add a bung if your application needs one. In that case, you would weld in a bung and then use a plug if you are not installing a sensor. Always confirm the bung thread size before ordering a plug.

• Inspect your exhaust for existing bungs before buying a plug.
• If you need to add a bung, a muffler shop can weld one in quickly.
• Keep a spare plug in your toolbox; they are small and easy to lose.

Do I need a gasket or anti-seize on a bung plug?

A gasket is highly recommended. Many bung plugs come with a copper crush washer that deforms to fill small imperfections and create a gas-tight seal. If your plug did not come with one, you can buy a copper or aluminum crush washer in the correct size. Anti-seize is also recommended, especially for stainless steel plugs in stainless steel bungs, to prevent galling and make future removal easier.

Apply anti-seize sparingly to the threads only. Do not get it on the sealing surface of the gasket, as it can cause the gasket to slip. If you are using a plug with a tapered seat (no gasket), do not use anti-seize on the seat; only on the threads.

• Copper gaskets are common for exhaust use; replace them if they are crushed or discolored.
• Anti-seize can affect torque readings slightly; reduce torque by about 10% if you use it.
• Never use Teflon tape on exhaust bung plugs; it melts and creates a mess.

Bung Plug Quick Reference

Thread size

Confirm your bung thread before ordering. Common sizes are M18 x 1.5 and M12 x 1.25. Measure twice, order once.

Torque range

M18 x 1.5: 18–22 ft-lbs (25–30 Nm). M12 x 1.25: 12–15 ft-lbs (16–20 Nm). Always check manufacturer instructions. Use a torque wrench and recheck after the first heat cycle.

Leak fix

Clean threads, replace gasket, apply anti-seize, and torque to spec. If threads are damaged, repair or replace the bung. A leak is usually a simple fix, not a major repair.

Relevant MagnaFlow and BRExhaust Products

• MagnaFlow 1980-1981 Chevrolet Corvette 5.7L Street Series Cat-Back Performance Exhaust System 16889: This cat-back system uses mandrel-bent 2.5in. main piping and 2 straight-through mufflers ending in a dual split rear exit with 2.625in. polished welded-on single wall turndown cut edge tips. If your system has an unused bung, a plug seals it without disrupting the mandrel-bent flow path.
• MagnaFlow OEM Grade Federal / EPA Compliant Universal Catalytic Converter 51506: This universal converter has a compact, low-mass design and stainless steel construction with single laser fusion welds to reduce leak potential. If you add a bung to a universal converter, use a matching plug and gasket to maintain the leak-resistant design.
• BRExhaust 2014-2017 Volkswagen Passat 1.8L Direct-Fit Exhaust System Kit 106-0820: This direct-fit replacement exhaust kit includes all gaskets and clamps needed for installation and uses precision mandrel-bent pipes for superior exhaust flow. If your replacement exhaust has an unused sensor bung, a plug is the correct way to seal it while keeping the included hardware for the rest of the system.

A bung plug is a small part with a straightforward job: seal an unused oxygen sensor hole. Get the thread size right, use a gasket and anti-seize, and torque it properly, and you will avoid leaks and future headaches. If you are unsure about your bung size, check your exhaust system or consult a professional before ordering.