The Yankee Hill Machine Nielsen Booster Assembly replaces the direct thread adapter on a YHM Sound suppressor or any other suppressor on the market which utilizes the 1-3/8"-24. They are designed to be used with handguns and barrels which require a Nielsen Piston spring. Note: DOES NOT INCLUDE PISTON FeaturesReplaces your Direct Thread Adapter17-4 Stainless SteelMatte Black Finish1-3/8"-24 Mount Thread Pitch