Stainless Works O2 Bung

Weld It In Once, Thread Sensors In Forever: The Fabricator's Go-To O2 Bung from Stainless Works

The Stainless Works O2SS is a weld-in oxygen sensor bung CNC machined from 303 stainless steel. Whether you are building a custom exhaust from scratch, relocating a sensor after a catback swap, or adding a wideband O2 port to a fabricated header, the O2SS gives you a permanent, precision-threaded 18mm bung that accepts any standard oxygen sensor. The lead-in chamfer makes sensor installation straightforward even in tight underhood or undercar spaces. Because it is machined from 303 stainless rather than mild steel or cast iron, it resists rust and corrosion at the temperatures common in exhaust applications and welds cleanly using standard TIG or MIG processes with stainless filler rod.

Key Features

• CNC machined from 303 stainless steel for corrosion resistance and clean weld characteristics
• 18mm thread pitch accepts all standard OEM and aftermarket oxygen sensors
• Lead-in chamfer on the threaded bore for easy sensor installation
• Semi-universal: compatible with any 304 or 303 stainless exhaust tubing or mild steel pipe
• Sold individually
• Made in the USA by Stainless Works

Specifications

What's Included

• 1 x Stainless Works O2SS weld-in oxygen sensor bung

About Stainless Works

Stainless Works is a US manufacturer based in Valley City, Ohio, producing premium 304 stainless steel headers, catback exhaust systems, mid-pipes, and fabrication components since 1990. Every Stainless Works product is designed and built in-house by American craftsmen using mandrel-bent tubing, TIG-welded joints, and precision CNC-machined fittings. The O2SS bung is a fabrication staple used by professional exhaust fabricators and home builders alike who want a clean, durable threaded solution for oxygen sensor placement.

Frequently Asked Questions

The O2SS is threaded to accept standard 18mm oxygen sensors, which is the most common thread size used by virtually all OEM and aftermarket O2 sensors including narrowband switching sensors, wideband lambda sensors (such as Bosch LSU 4.9 and LSU 4.2 types), and air/fuel ratio sensors. Before welding, verify your specific sensor uses an 18mm thread.

Yes. While the O2SS is machined from 303 stainless steel, it can be welded to both stainless exhaust tubing and mild steel pipe. When welding stainless to mild steel, use a 309 filler rod to bridge the dissimilar materials and ensure a clean fusion weld with adequate penetration. Standard 308 filler is appropriate for stainless-to-stainless applications. TIG welding is preferred for a watertight, corrosion-resistant bead around the bung.

For upstream (pre-catalyst) sensor positions, weld the bung into the collector or exhaust pipe within 12 to 18 inches downstream of the exhaust port. For downstream (post-catalyst) sensor positions used for OBDII catalyst monitoring, weld the bung after the catalytic converter outlet. Always orient the bung so the sensor opening faces no more than 10 degrees past horizontal to prevent condensation from pooling in the bung and damaging the sensor element. Avoid positioning directly at the bottom of the pipe (6 o'clock) for the same reason.

Yes, Stainless Works recommends applying Never-Seez (or equivalent copper-based anti-seize compound) to the oxygen sensor threads before installation. The high exhaust temperatures cause metal-to-metal galling over time, making sensor removal difficult or impossible without anti-seize. Apply a small amount to the sensor threads, avoiding the tip of the sensor element, and torque to the sensor manufacturer's specification (typically 33 to 44 ft-lbs for 18mm sensors).

Thread in a sacrificial bolt or a tap wrench into the bung before welding. This protects the 18mm threads from weld spatter and heat distortion during the welding process. Tack the bung in position first at two to four points, then complete a full perimeter weld around the base. Allow the weld to cool before removing the sacrificial insert and cleaning the threads with a tap if needed.

Yes. The 18mm thread pitch on the O2SS accepts wideband Bosch LSU 4.9 and LSU 4.2 sensors used with Innovate, AEM, PLX, and virtually all standalone wideband controllers and data loggers. For permanent wideband sensor installations on a dedicated track or dyno car, welding in an O2SS bung is the correct approach versus using a clamp-on or snap-in bung, which can loosen at sustained high exhaust temperatures.

For inline-4 or straight-6 engines with a single exhaust system, one bung per pipe location is correct. For V6, V8, or flat-6/8 engines with dual exhaust (two separate exhaust banks), you typically need one bung per bank for each sensor location. OBDII applications generally require one upstream and one downstream sensor per bank (four total on a dual-bank V8). Order additional O2SS bungs based on your engine configuration and sensor layout.

The lead-in chamfer is a machined angled surface at the entry of the threaded bore that guides the oxygen sensor tip into the threads without cross-threading. On installed bungs that are in a difficult-to-reach location under the vehicle, this chamfer is particularly useful: it allows you to feel the sensor engaging the threads correctly before applying torque. Without a chamfer, threading a sensor into a bung blind or at a slight angle risks stripping the 18mm threads, which are very difficult to repair in a welded bung.

Installation Instructions

1. Determine the correct location for your oxygen sensor on the exhaust pipe. Ensure the bung will be accessible for sensor installation and service, and orient so the bung opening is not pointing straight down.
2. Mark the center of the bung location on the pipe. Using a center punch, mark the pipe before drilling to prevent the drill bit from walking.
3. Drill a pilot hole, then step up to the appropriate size drill bit for the bung's outer diameter. Clean any burrs from the drilled hole with a file or die grinder.
4. Thread a sacrificial bolt or tap wrench into the O2SS to protect the threads during welding. Fit the bung into the drilled hole.
5. Tack weld the bung in position at two to four evenly spaced points, checking alignment before completing the full perimeter weld.
6. Complete a full TIG or MIG weld around the base of the bung. For stainless-to-stainless welds, use 308L filler. For stainless bung to mild steel pipe, use 309L filler.
7. Allow the weld to cool completely. Remove the sacrificial insert and inspect the threads. Chase with an 18mm x 1.5 tap if any spatter entered the bore.
8. Apply Never-Seez or copper-based anti-seize compound to the oxygen sensor threads. Install the sensor by hand until finger-tight, then torque to the sensor manufacturer's specification (typically 33 to 44 ft-lbs).
9. Connect the sensor wiring harness and verify routing is clear of exhaust heat. Start the vehicle and inspect for exhaust leaks around the bung weld.

WARNING: This product can expose you to chemicals including lead and phthalates, which are known to the State of California to cause cancer and reproductive harm. For more information go to www.P65Warnings.ca.gov.