Used in custom applications to reduce exhaust sensor thread from 18mm to 12mm so late H-D models, equipped with 12mm O2 sensors, can be used with earlier 18mm sensor style exhausts. Note: For Touring models an extra extension cable is required, which is available as 578303 bung set & cable extension kit.
Fits: > 12-17 Softail, Dyna (with pre-2012 exhaust) (NU); Other applications were 12mm sensors are used for 18mm holes