The PPS/SHS Shock Transfer System (STS) is the best remedy for preventing version 2 airsoft gearbox failure. If you haven't heard already, the metal v2 gearbox in many airsoft guns are prone to front end cracking and eventual breakage. The cause is multiple high force strikes of the piston.
The idea of the STS is to brace the front of the V2 metal gearbox against the M4/M16 upper receiver. The only constraints are that the M4-varient upper receiver must be metal and the hop-up must be one-piece. Luckily most M4-variant airsoft guns comes with one-piece hop-up.
The Modify Well Lock Bracer improves on the design of the STS. In the Modify Well Lock Bracer Review, we compare the improved bracer to the original.
The STS comes in a clear plastic package with no instruction (see photo below). Luckily, there are various videos online that shows you how to install it. This article will also fill that gap.
The STS has two hex screw on top. They are for fastening the shock transfer pins (silver). The shock transfer pins are spring tensioned so that STS will fit different M4 receivers. The allen hex wrench for the screws are not included in the package. So you'll have to dig it out of your own toolbox or buy one from the store. The hex screw size is 2mm.
Once you have the right hex wrench, separate the upper and lower receiver of you M4-variant gun. This review won't cover disassembly for your airsoft gun. But instructions for M4-variant airsoft guns are all over the Internet.
With the receivers separated, you'll end up with the v2 gearbox on the lower receiver and the inner barrel with the hop-up on the upper receiver. The aluminum STS fits loosely around the one-piece hop-up with the silver pins toward the front of the upper receiver.
Once the STS is in place, reassemble the upper and lower receiver. And fasten the two receiver together with the receiver pin. Through the M4 magazine well, you should see the two hex screws. Use the 2mm hex wrench to tighten the two screws to stiffen the STS pins.
The instruction in the last paragraph is general instructions for most M4 airsoft guns. But I actually had quite a bit of trouble installing the STS in the APS ASR106 Mini Patriot M4. The details for this installation is covered in the next section, titled "STS in APS M4 Receiver".
There are several differences in the APS M4 receiver that made STS installation a bit different than standard installation.
First of all, the APS gearbox is typically a electric blowback (EBB) model. Therefore, the EBB plate strikes the STS. The solution I chose is to disable the EBB on the gearbox. (See "Disable Electronic Blowback on APS Airsoft Guns" thread).
Second, the APS M4 receiver magazine well has more metal and is more sturdy than the standard M4 receiver. Therefore, the magazine well actually blocks the hex screws (see photo below). The correct solution is probably to file out the receiver to reveal the hex screws.
But I didn't want to modify the APS M4 receiver. Instead, I manually adjusted the spring pins on the STS until it fit snugly between the upper receiver and the APS v2 gearbox without any free play. Then I put the two receiver together with the pin.