The AEG piston is one of the most important parts of the gearbox. It also undergoes the most wear in a gearbox and needs to be properly matched to the type of build (torque or speed). Almost all airsoft pistons are universally compatible with all AEGs, with a few working in specific designs. Zephyr Sports offers some of the best pistons we have found on the market, so if you have a stripped piston, check out our reinforced models!
Half Tooth / Full Tooth
Pistons largely fall into 2 different categories, full tooth and half tooth. Most gearboxes and gear systems are meant to use full tooth pistons. Using a half tooth piston on regular gears only uses half the provided surface area causing the piston to miss align or strip out faster. Half tooth pistons are designed for a special set of airsoft gears where the sector gear's teeth are the same diameter (regular gears have a larger and smaller set of teeth diameters). These gears are often helical gears or made by a special manufacture and need half of the teeth removed so that the 2 sets of gear teeth don't hit the piston.
All airsoft pistons have at least 1 metal tooth at the very end. The last tooth on the piston takes the largest amount of force from the spring and is the most likely to break so airsoft manufactures made it metal. Some pistons will have half of the teeth be made of metal, this design is meant to be used with torque builds that put a lot of pressure midway through the cycle. there are even pistons with a full set of metal teeth that need to be super glued in place or they will break the very end piece. It is very important to test whether a piston will work in an airsoft gun with the gearbox shell closed and the sector gear inside. Some manufactures with change the height of the teeth of the sector gear that may cause them to push up on piston as it slides back and break the piston. Alternatively some manufactures place the rails the piston slides along too low causing the same problem.
The vast majority of airsoft pistons are either ABS or polycarbonate. ABS pistons are cheaper, lighter and much more likely to strip out. Polycarbonate pistons are much more resilient than ABS models and are structurally sound enough for most airsoft guns. Some newer pistons are made from Azimuth. These pistons are super light weight and shock resistant, but are otherwise brittle and can wear down easily if not installed correctly. Aluminum pistons have recently come into existence, using steel teeth they are market as the strongest pistons available. While they will not fracture, break or snap, an aluminum piston may scratch and wear down the inside of the gearbox or warp. Changing to an aluminum piston changes the usual plastic on metal contact to metal on metal, which if not lubricated often can cause scratches on the cylinder, and gearbox shell. The repeated trauma can also make the aluminum piston warp or deform from the repeated firing of the AEG resulting in even worse gearbox malfunctions.
While plastic and polycarbonate piston heads are ok, machined aluminum pistons head provide a much better seal in the cylinder. This is because the plastic piston heads are cast, rather than machined meaning they have a greater margin for gaps and deformations. Most polycarbonate and plastic piston heads are also thin and made from several pieces, causing them to come apart easier or break.
Piston Head Ports
Piston head fall in to 2 major categories, ported and non-ported piston heads. Non-ported piston heads force the O-ring out and create a strong seal when fired. They also, however, keep that seal when the piston is being pulled back. This causes either the seal to break from the low internal pressure being created or air to get slowly sucked into the cylinder from any gaps or the air nozzle. Ported piston heads create a seal while firing because as the piston head moves forward it compresses the air, the air moves into the ports and force the o ring to expand. The greater the pressure, the greater the seal from the ported piston head. Once fired, a ported piston head loses its seal. When the piston is being drawn back air flows against the back of the piston head and out through the ports, letting the piston move back easier. It is generally believed that although non-ported piston heads have better compression and are cheaper, ported piston heads perform almost as well and allow for a high cyclic rate.
Silent / Mushroom / Stealth Piston Heads
These piston heads are designed with a very unusual shape to reduce the sound an AEG makes as it fires. Rather that a typical flat piston head, mushroom piston head have a bulge in the center that channels the air at the end of the firing stroke and is supposed to reduce the sound signature of the AEG.
Piston Head Bearings
Many after market piston heads and some stock ones, have bearings that are held to the inside of the piston. These bearings are pressed into the piston by the spring and allow the spring to spin or twist as it is compressed. These bearings are important because without them a spring can get caught and cause a jam in the gearbox or snap.