Another ICS Class video shows how to replace the piston in the ICS L85/L86 AEG Series. This takers more time as you will need to thoroughly open the gearbox in order to get the piston and replace it. Our suggestion is that whilst you're there replacing the gearbox, better check the rest of the internals such as the gears to ensure that they are in good shape and lubricated properly before your close it back.