Zinc plated steel. Fits 18mm x 1.50 sensor bungs. Required with non-O2 (oxygen) sensor equipped 86-up H-D models that have an exhaust system installed with stock threaded sensor plug holes. Can also be used on H-D models when O2 sensors are no longer required.
Fits: > 86-06 H-D models (with exhaust system equipped with O2 sensor bungs)