O2 Sensors : For all 2007-2018 Sportster, O2 bungs are essential in order to keep the correct air/fuel ratio from your ECM. 
The only exception is if you are running o2 eliminators.

These exhaust reducers fit the larger 18mm threaded ports, reducing down to 12mm for later model bikes. 

Fitment : When fitting the exhaust reducers , insert and start threading by hand. Then tighten securely.

IMPORTANT NOTICE – D-Star Racewear and Customs / Messrs Johnson accept no responsibility for any mechanical issues that your vehicle whilst using a custom exhaust.
With the correct fitment and fuel tuning (if you have an fp3 please see the product info for setup) these will operate to perfect conditions.

It is your responsibility to ensure your bike is road legal at all times and all parts meet local laws.
Vehicles should be have all parts and bolts checked after the first 50 miles of use, then every 100-150 miles after to ensure everything is fastened correctly and in good operational order.