Ah yes.  We will be revisiting my beloved Elite Force HK416 CQB by VFC this week with a guide on how to install a MOSFET unit inside your rear-wired M4 Airsoft AEG.   
She has been in pieces for far too long now (for the better part of a year, I'm ashamed to admit).  It all stemmed from a stripped piston last year during the Association of Competitive Airsoft Expo that I participated in with a few fellow members from my Airsoft team, Level X. 
Since I had to open up the gearbox anyway, something I had refrained from doing unnecessarily since tuning my HK416 a year or more prior to that, I decided to take a thorough assessment of the wear & tear on the internal parts of the gearbox to see if anything else needed repair or replacement.  Eventually, when I got to the trigger switch, or wire harness as some call it, I found that one of my copper contact plates had burnt up pretty good over time, undoubtedly due to the fact that I had been running 11.1V Li-Po battery packs exclusively in that gun for over a year and had used it quite often (the Elite Force HK416 CQB makes for a phenomenal workhorse primary).
While I don't pretend to be a true expert here, I'm of the opinion that the best way to prevent this wear and tear on the trigger contacts while still being able to run Li-Po battery packs is to install a MOSFET computer unit in your AEG.  So since this was, and still is, my favorite of the M4 variants in my little collection, I felt it deserved a MOSFET and was worth the effort to install one.  I will preface the rest of this blog by saying that this installation process has turned into a much more complicated project than I had originally anticipated, but I plan to see it through until I can get my HK416 back up and running because, like I said, it's worth it. 

This isn't my first rodeo with installing a MOSFET, but in the past, I've done it on guns using a full, fixed stock, like my Marui M14 or Classic Army SR25.  When I initially made the decision to take on this project, I failed to take the HK416's buffer tube into account, since this Airsoft AEG is wired to the back.  Oh well.  Adapt and overcome.  Otherwise you will lose.  Since I have an allergy to losing, I wasn't about to give up.  

So, initially, I wrote this blog about my MOSFET-in-the-buffer-tube mod.  Keep in mind that it only covers what I did to fit the MOSFET in the buffer tube and still allow for you to be able to connect all the wires and secure the buffer tube to the back of the receiver.

This modification is not for the faint of heart.  Attempt at your own risk.

Click on the images for a larger view of what I did.  But basically, I used tiny, 20 or 22 awg (gauge) wire for the trigger circuit.  The MOSFET only allows a small amount of electrical current to be routed to through the trigger contacts, thereby preventing them from getting burned up like you saw in my image before the jump, so a large gauge wire is not necessary.

I removed the old wire by heating the contacts up with my soldering iron and then used the red & white wire you see below and re-soldered the contacts using those, but you can actually just use one color because all that is happening with the trigger is completing a circuit when the trigger is pulled and the contacts are bridged.

I have seen others rewire the gearbox so that these wires are on the outside of gearbox, running along the side of it, but that makes it difficult to get the gearbox in and out of the lower receiver, which is especially annoying when you're trying to diagnose a gearbox problem that requires you to repeatedly assemble and disassemble the gearbox to figure out what the problem is.  
Not really related to this post, but I cleaned up all that old grease that came with the gun and re-greased the gears with White Lithium Grease with Teflon in it.
 The downside to wiring the gearbox my way is that you'll have to be brave and permanently modify your gearbox in two spots to allow for the other two wires needed to enter and exit the gearbox as well. More photos to illustrate this will come below.  The first spot is at the back of the gearbox, which you can see above.  I don't have a photo of the before shot, but trust me when I say that there just wasn't enough room for all four wires to get through the hole that was there.  So I widened it with my Dremel rotary tool.  If you don't have a Dremel, get one.  If you cannot afford one, get a set of files.  It will take you more time, but it's still doable.

The other hole that I modified on the gearbox can be seen below. 
 Normally, the red wire exits the gearbox to connect the motor through one of those two smaller holes (one on each side of the largest, motor shaft hold), and the black wire exits the other hole (we'll say the one on the right for this example).  This requires the wire to pass underneath the motor pinion gear that sits just inside the gearbox and spins.  However, because of the large gauge wire that I use, along with the fact that I've also got that smaller wire for the trigger in there as well, there is not enough room for the motor pinion gear to clear the wire while it spins.  This is obviously a big danger area because you certainly don't want to have your pinion gear start spinning and then tear up your wiring.  Not sure, but it couldn't open up the possibility of start a small electrical fire inside your gearbox, which would just add insult to injury.

So my solution is seen below, where I widened the hole on the left so that both of the large-gauge wires can exit that one hole to get down to the motor, without any of those two wires having to pass underneath the motor pinion gear inside the gearbox. 

Here is the end result of that so that you can see how it all comes together.  You can click on the image to open inside a new tab at a higher resolution for a more detailed illustration. 
So the next part can be a bit tricky if you don't do it a certain way.  It is the process of getting the correct length of wire to connect the motor.  When you begin this process, make sure that you start with more wire length in each color than you think you will need, just in case you end up needing a little extra length somewhere down the line.  This is a prime example of when it's MUCH better to have to waste a little bit of extra wire than it is to get into a situation where you've already wired more of this system for the new MOSFET unit, only to find out that you don't have enough length and will have to start over.  That would SUCK!!!

So, in the photo below, you can see that I have already attached one of the motor connectors to the end of the red wire, while the black wire is still without its connector.  It is much easier if you wait to attach the motor connectors until AFTER you've assessed the amount of length you will need to connect the motor.  If you add your connectors first, like I've down with the red wire, you will then need to do a much more precise job of measuring ahead of time. 
The best way I know how to set this up is to take the grip with the motor inside of it and seat it at the bottom of your gearbox shell, feed the wires through the grip like you normally would any AEG like this and then cut the wires to the length you will need, allowing of course for the motor connectors to be attached.  It's also good to keep in mind that when the motor plate is screwed into place on the bottom of your grip, the motor will get pushed up higher into the grip, so you will want to make your measurements on the wire length with the motor pushed up a bit higher into the grip so that you don't end up with too much extra length of wire inside that grip, as you may not have room for it. 
Here is just another view of how I set this up so that I could get the correct wire length running down to the bottom of the motor. 

So once you cut your wires to the right length for your motor and then attach your motor connectors, you're ready to start putting the gearbox back together. However, I'm not sure about other gearboxes off the top of my head, but with the Elite Force VFC HK416 CQB gearbox, there are two pegs that the gearbox shell was made with that press down on the wires inside their dedicated wire channel to help keep them in place when the gearbox is closed.  However, with all the extra wiring used for this modification, these pegs will need to be removed in order to close the gearbox all the way.  Another job for your Dremel or files.  In the image below, I'm referring to the two pegs that are sticking up along the backside of the gearbox shell's wall. 
Below is an after-shot, where I have removed the pegs with my Dremel tool. 
Once you remove those guys, you will not be able to close the gearbox shells completely.

 One last item I would recommend is cutting the wiring hole in the top of the grip to allow the wires to feed much more freely from the gearbox down into the grip.  Otherwise, the wire jackets may get cut or torn on sharp edges inside there, leaving portions of your wire expose, which could result in a short circuit and/or electrical fire if the right elements are in place to do so.

I will finish up the rest of this process tomorrow.  This part of the process alone is enough to keep you busy for a couple hours, especially if this is your first time attempting this.  If you do decide to try this on your own guns, make sure you know your way around the inside of a gearbox pretty well.  If you've never tried to take apart a gun before, don't bother with this mod.  There is a good chance you will just end up ruining your gun.  If you have any questions as you're working through this process, feel free to leave me a comment in the section below and I'll be glad to help you out.
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