Sold as singles or choose quantity 2 for twin cylinders.
These are required when deciding to remove your stock 18mm O2 sensors. This would be in a situation where Rottweiler Performance Fueling Dongles or a Powercommander V is being used and the stock narrow band O2's have been rendered unused. Titanium provides the best corrosion properties compared to stainless steel and mild steel.
Bikes that use 18mm O2 plugs: