The air nozzle, cylinder head, cylinder and piston head make up the major seals in airsoft gearboxes. Adding the right airsoft upgrade of these can improve an airsoft guns consistency, efficiency and fps. cylinder heads are gearbox specific, a V2 cylinder head for V2 gearboxes and V3 for Version 3 gearboxes. Air nozzles are model specific, with M4 air nozzles meant for M4s and AKs for AKs. Cylinders come in different cuts to match the barrel length used in the AEG.
A cylinder head seals the end of the cylinder and holds the assembly in place by fitting into two mounts on the gearbox shell. It acts as a guide for the tappet plate and air nozzle to make sure that they line up with the hop up unit. Cylinder heads are version specific, meaning each version corresponds to the gearbox version. Some cylinder heads use two O-rings instead of one to increase the seal between it and the cylinder. Higher quality cylinder heads are machined better and provide a better seal, and a more solid fitting than their stock counterparts.
Each air nozzle is model specific, so an M4 air nozzle will only fit an M4 and not work with an AK. Air nozzles attach to the tappet plate and are used to load BBs into the hop up unit every time the gearbox starts to cycle and the tappet plate moves forward and back. The air nozzle creates a seal between the cylinder head and the bucking inside the hop up unit and funnels the air out to propel the BB through the barrel. High performance air nozzle have an O-ring on the inside of the tube that rides along the cylinder head and keeps a constant seal. Stock versions of air nozzles rely on tight fits to keep the seal and may leak air out when firing.
Cylinders come in two different types and three different cuts. There are bore up cylinders that have a tighter inner diameter meant to create a better seal and be used with a bore up kit to increase the range and fps of an airsoft AEG, or standard cylinders that are found stock in most airsoft AEGs. Cylinders are often made of brass or aluminum and may have a special coating on the inside to improve the air seal. while the coating does help, using a good grease to lubricate the cylinder works better. Cylinders come in three cuts referring to the location of the port on the side. These cuts exist to affect the volume of air compressed compared with the volume of air in the inner barrel. Ideally, the ratio from cylinder to inner barrel is between 1.5:1 and 1:1. Too low and the BB may not make it out of the barrel, too high and the AEG may become highly inaccurate and inefficient. 'O' cylinders or full cylinders are just a straight tube that does not have a port, theses are best paired with long airsoft barrels such as the 509mm M16 barrel. 3/4 cut cylinders have a port that starts 3/4 of the way back on a cylinder. These are best paired with an airsoft barrel between 360mm to 509mm. 1/2 cut cylinders have ports that start 1/2 way down the cylinder and are best paired with AEGs that have a barrel less than 360mm.