Bingo Airsoftworks shows how to install this kit they made for the L85 AEG... "This video demonstrates how to install the G&G/Army HPA Engine gearbox replacement kit. The kit allows you to easily upgrade your G&G or Army L85 to use an HPA engine such as the PolarStar Fusion Engine or the Wolverine SMP Engine. The video shows how to remove the AEG gearbox and install the kit, for both the Fusion Engine and the SMP versions."