The O2 Sensor Shuffle: Why Your New Headers Have Extra Holes
If you’ve recently unboxed an aftermarket exhaust system for your motorcycle, you might have found yourself staring at an unexpected "extra" hole in the pipe.
In the world of tuning, the stock O2 (Lambda) sensor or sensors are often eliminated to allow for a more precise fuel map. However, as motorcycle emissions standards evolve, the hardware on your headers is changing, too. Here is the breakdown of why those bungs are there and some simple solutions.
The Evolution: From M18 to M12
Not long ago, "race" headers almost exclusively featured a large M18 x 1.5mm bung.This size is the industry standard for Wideband O2 sensors (like those used with a Dynojet Autotune or professional dyno equipment). Because OEM sensors are much smaller (M12 x 1.25mm), manufacturers used to include a reducer adapter. This gave riders the flexibility to run the stock sensor, a wideband sensor, or plug it entirely.
Recently, we have seen a shift. Many new aftermarket headers are ditching the M18 bung and moving toward one or two M12 x 1.25mm bungs depending on the application. This reflects the industry's move toward more compact, integrated electronics and of course cost savings.
The Euro 5 Factor: Why Two Bungs?
The biggest source of confusion for modern riders is the sudden appearance of a second bung. This is a direct result of Euro 5 emissions standards.
To meet these strict regulations, many newer bikes utilize a more complex monitoring system that may require an additional sensor to track catalyst efficiency. When exhaust manufacturers build a "one-size-fits-all" header for a global market, they include bungs for every possible sensor location required by law.
The Problem
Many of these exhaust kits only ship with one bung plug. If you are tuning your bike and eliminating both sensors, or in the US, eliminating the one sensor, you’re often left with one hole open and no hardware to fill it. Many riders are caught off guard when the header they purchased only has the one plug and they need to source a second.
Sizing Guide: Getting the Right Plug
If you’re left with an open bung after your tune, you need to seal it properly. An exhaust leak at the bung will cause popping, poor performance, and can even damage your engine over time.
|
Sensor Type |
Thread Size |
Common Use |
|
Modern OEM / Euro 5 |
M12 x 1.25mm |
Standard on most new dualsport, adventure, sportbikes and cruisers. |
|
Classic/Wideband |
M18 x 1.5mm |
Used for data logging and older performance headers. Some “high performance” brands still send their headers out with the M18 bung. |
Pro-Tips for Plugging the Gap
Anti-Seize is Essential: Exhaust heat cycles are brutal. Always apply a small amount of high-temp anti-seize to the threads of your plug. If you ever decide to reinstall your sensors or add a wideband for data logging later, you’ll be glad you did.
Copper Crush Washers: Don’t just rely on the threads to seal. Use a copper washer to ensure a gas-tight fit.
Terminology Check
If you see the term “Lambda Sensor” used and are not sure what that is, it is simply the technical/engineering name for an O2 Sensor. European manufacturers (like BMW, KTM, or Ducati) and performance tuners almost always use "Lambda."
Still not sure which plug your system needs? Reach out to us atTMBR MOTO - tmbrmoto@tmbrmoto.com - we’ll help you get the right hardware so you can get off the lift and back on the road.
