Oxygen Sensor Bung Plug Sizing: Match Thread and Seal

By MagnaFlow Exhaust, Mufflers & Cat Converters - Quality | Published: 2026-10-16

Category: How-to Guides

Learn how to size an oxygen sensor bung plug correctly by matching thread, seat type, and sealing method. Includes a step-by-step process to plug an O2 bung without leaks.

Quick answer

To size an oxygen sensor bung plug correctly, first identify the thread size and pitch of your bung, then choose a plug with the same thread and the correct seat type (flat, tapered, or gasket-sealed). The most common O2 bung thread is M18 x 1.5, but you must verify yours before ordering. Always use a new sealing washer or thread sealant rated for exhaust temperatures.

Identify your bung thread size and pitch

The thread size and pitch are the most critical dimensions. An M18 x 1.5 plug will not seal in an M18 x 1.0 bung, and forcing it will damage the threads. Use a thread pitch gauge or calipers to measure the outside diameter of the bung's threaded hole and the distance between thread peaks. Common O2 bung threads include M18 x 1.5, M12 x 1.25, and M10 x 1.0. If you cannot measure directly, check your vehicle's service manual or the bung manufacturer's specifications. Do not guess.

Determine the seat type: flat, tapered, or gasket

Oxygen sensor bungs typically have one of three sealing surfaces: a flat machined face, a tapered (conical) seat, or a flat face that relies on a gasket or sealing washer. The plug you choose must match this seat type. A tapered plug in a flat bung will not seal, and a flat plug in a tapered bung may bottom out before sealing. Inspect the bung opening with a flashlight. If the sealing surface is a smooth, flat ring, you need a flat-seat plug. If it is angled inward, you need a tapered-seat plug. If there is a recess for a gasket, use a plug with a matching gasket or a sealing washer.

Choose the right sealing method for exhaust heat

Exhaust temperatures can exceed 1,000°F, so standard rubber O-rings or nylon washers will fail. Use a copper or aluminum crush washer, or a graphite-based exhaust sealant. Some plugs come with a pre-applied thread sealant, but verify it is rated for exhaust. If using a sealing washer, ensure it fits the plug's shank diameter and seats properly against the bung face. A bung plug sealing washer that is too large may not compress evenly, and one that is too small may not cover the sealing surface. Always follow the plug manufacturer's recommendations.

Install the plug without cross-threading

Start the plug by hand to avoid cross-threading. Turn it counterclockwise until you feel the threads engage, then turn clockwise. Once finger-tight, use a wrench to snug it. If you are using a crush washer, tighten until the washer compresses slightly. Do not overtighten, as this can strip the bung threads or distort the sealing surface. For tapered-seat plugs, tighten until snug; the taper creates the seal. After installation, start the engine and check for leaks around the plug. If you detect a leak, remove the plug, inspect the threads and sealing surface, and reinstall with a new washer or sealant.

Steps to plug an unused O2 bung

Measure the bung thread: Use a thread pitch gauge or calipers to determine the thread size and pitch. Common sizes are M18 x 1.5, M12 x 1.25, and M10 x 1.0. Write down the measurement. Checkpoint: If you cannot measure, consult a professional or the bung manufacturer. Do not rely on visual estimation.
Inspect the seat type: Look at the sealing surface inside the bung. It will be flat, tapered, or designed for a gasket. Choose a plug with a matching seat. Common mistake: Assuming all bungs are flat-seat. Tapered seats require a tapered plug.
Select the plug and sealing method: Order a plug with the correct thread and seat. Choose a copper or aluminum crush washer, or an exhaust-rated thread sealant. Avoid rubber or nylon washers. Checkpoint: Verify the plug's temperature rating exceeds 1,000°F.
Install the plug carefully: Hand-start the plug to avoid cross-threading. Tighten with a wrench until snug. If using a crush washer, tighten until it compresses slightly. Do not overtighten. Common mistake: Over-tightening, which can strip threads or damage the sealing surface.
Check for leaks: Start the engine and let it idle. Listen and feel for exhaust pulses around the plug. If you detect a leak, remove the plug, inspect the threads and washer, and reinstall with a new sealing component. Next step: If the leak persists, the bung threads may be damaged and require repair or replacement.

Product facts

MagnaFlow 6 in. x 6 in. Round Straight-Through Performance Exhaust Muffler 14159: The MagnaFlow 6 in. x 6 in. Round Straight-Through Performance Exhaust Muffler 14159 features a reversible, universal 3/3 in. diameter inlet/outlet layout and a 6x6x6 in. round body shape, with a free-flowing straight-through perforated stainless steel core wrapped in stainless steel mesh and acoustic fiber fill. This muffler is a universal component that can be used in custom exhaust builds, but it does not include an oxygen sensor bung. If your build requires an O2 sensor, you will need to weld in a bung separately. The straight-through design and stainless construction are typical of performance exhaust parts that may require additional fabrication for sensor integration.