The A&K SR-25 Spacer Kit comes with a delrin spacer which gets pressed into the front of the Fusion Engine's cylinder to make the Version 2 engine the same length as the original extended Version 2 gearbox used in the rifle. Also included is a M5x0.8mm nut which is placed inside the rear of the lower receiver for the stocks bolt to thread into instead of the rear of the engine. This kit coupled with the A&K Masada Nozzle (purchased separately) allows the engine to interface with the rifles original M4 type hop-up chamber.
