Concern For MechBox Well-being

[sgtscarfy]

Ok so I recently upgraded my tm m4a1 mechbox with the following:

Piston Head: SystemA Silicon Piston Head w/Bearing
Cylinder Head: Guarder
Nozzle: SystemA
Spring Guide: SystemA w/ Bearings
Spring: PDI 120 & soon a Prometheus MS100SP
Inner Barrel Prometheus 6.03mm BS (363mm)
Hop Up: G&P Metal

This was my first time tearing the mechbox apart but I did alot of research with mechbox.com and other sources. Ok so i spent about an hour to properly shim it(i used http://www.geocities.com/wajoegween/aegshim.htm as a guide). So I put it all together and it seems to shoot fine with zero misfeeds or other problems(~400 rounds). Now the only concern is that when i fire i hear a faint "springy" sound after every shot. Its hard to describe... Like a spring vibrating... ok so i think it might be the gears but you guys are the experts! Any ideas? Should i be concerned for my MechBox blowing up anytime soon?

[666]

Sound can change when you replace internal parts. Plus you haven't done any upgrades to the gears according to your post.
You are running PDI 120 spring which is a little bit higher than stock so your mechbox should be fine. If your gun already went through around 400 shots and didn't stop working it should be fine.
Also, getting a set of metal bushings and shims might be a good idea. Stock bushings should work fine with soft spring but if you want to be on the safe side, get a set of metal ones.

[sgtscarfy]

Oh i forgot to add the shims and metal bushings in the post... ya i just wasnt sure if i did something wrong in the upgrade

[Omi-san]

I'm no expert but...

Did you put the right end of the spring on the spring guide(tight coils)? Did the PDI 120% spring seem to have more tension than the stock one?

It seems to me that if the spring vibrates, it must not be under enough tension when the piston is forward.

[sgtscarfy]

The spring was put in properly. I took my time and made sure i put the tight coils on the spring guide, cause i didnt want to rip it apart again. The spring definately had more tension than the stock TM one. The sound i believe to be the gears meshing, i kinda noticed it when i was shimming the gears and testing them.

[ThunderCactus]

lol, my G3 does the same thing, it's because it has an aluminum piston head and there are the gearbox shell itself resonates with a loud 'PING' with every shot. Makes my gun very easy to recognize.
But TM mechbox shells are pretty weak, you may want to upgrade to a G&P or hurricane shell.

[CDN_Stalker]

PDI 120 could be a bit higher than stock, or it could be a LOT higher than stock. Better to get it out and put in a Prometheus 110 or 120 spring, your mechbox will be happier (my PDI 150 spring ......... 350fps.........busted my stock TM MP5 mechbox in two months, and my Hurricane reinforced mechbox after two years). Also, the PDI springs are stiffer than others and you'll probably find slower rate of fire and less battery life, especially if you are using a mini battery in your M4A1.

[sgtscarfy]

Is PDI really that bad? I hear good and bad things... why is the PDI so much harder on the mechbox when the Prometheus spring is stronger? If i was to be required to upgrade my mechbox and another spring, i prob shoulda just got a SystemA upgraded mechbox. Would have been easier and cheaper. Well im trying to remain CQB(<345ish fps) with this m4 so would a Prometheus M100SP be too much? I was looking on the spring guide from Illusion but not every gun is the same plus i dont want to go to a better mechbox. Im not made of money...

[Omi-san]

As far as I know...

Prometheus and Systema springs are non linear or irregular. Which means that the coils don't have the same distance between them. It's made like this to function better inside the mechbox. The quality control is also much better.

PDI springs are linear and have bad quality control. The rating on the package is apparently not a reliable indication of the spring's strength.

[CDN_Stalker]

Originally Posted by sgtscarfy

Is PDI really that bad? I hear good and bad things... why is the PDI so much harder on the mechbox when the Prometheus spring is stronger? If i was to be required to upgrade my mechbox and another spring, i prob shoulda just got a SystemA upgraded mechbox. Would have been easier and cheaper. Well im trying to remain CQB(<345ish fps) with this m4 so would a Prometheus M100SP be too much? I was looking on the spring guide from Illusion but not every gun is the same plus i dont want to go to a better mechbox. Im not made of money...

A Prometheus 100 spring should be all you need. Don't rely on Illusion's spring chart too heavily, it is largely one study and uses a single type of gun with lots of upgraded parts. If you want simple, consider rules of thumb. Avoid all % springs. It's largely just PDI that does this, shipping out new employee's practice springs (a joke but I'm not fully sure that ISN'T the case). Other springs are rated in meters per second, so consider the numbers to be in groups of 30fps and it'll give you a ballpark fps number. So a 100 should put you around 330fps, a 110 to be around 360fps, a 120 to be around 390fps. Remember, ballpark only. Add in a bearing spring guide will increase your consistancy, but also add about 15fps, as well as make your piston hit the cylinder head a bit harder.

Regarding PDI springs, they are made from stiffer wire (and have no set length to them, I've measured 150% springs to range form 6 3/8" long to 7" long), but also they only have compressed coils at one end. Systema and Prometheus both have compressed coils at one end, but also the middle.

Here's a good example of two upgraded MP5s:

My SD has a PDI 150% spring in it, bearing spring guide, metal bushings, Hurricane reinforced mechbox and 330mm stock TM barrel. Last time I chronied it when it was healthy was last year and I sat around 387fps with 0.20g BBs. It uses an EG 700 motor and even with my 8.4V 3300mAh battery, it has a medium rate of fire, not very fast at all. Velocity went from 351fps with just the spring, to 365 with the bearing spring guide, to 387fps after adding a 10cm longer inner barrel.

My other MP5, which I've been using 80% of the year, has a Prometheus 110 spring in it, Guarder tappet plate, EG700 motor, stock mechbox, etc. Only velocity increasing part is the spring. It regularly chronies at 365fps and only has the stock 229mm innner barrel. It has a MUCH higher rate of fire than my SD, even when using a simple 8.4V 600mAh battery.

Nutshell, PDI springs suck. They comes in varying lengths, resulting in extremely inconsistant velocities. My reasonable PDI spring busted both the stock mechbox and a reinforced one. My stock mechbox with 110 spring only is still doing fine after lots of shooting this year.

[TokyoSeven]

I was experiencing a twang like sound everytime I fired after an upgrade was done. Eventually my aeg stopped firing after less than 200 rounds. As it stands the screw that was holding the piston to the piston head had come off on the inside of the piston and fallen into the spring and jammed it. The twang sound was actually something rattingling everytime I fired. This may not be your case but it doesnt hurt to check.

[sgtscarfy]

Alright well im going to get a Prometheus MS100SP. I figure i can be smart and ditch the $20 PDI instead of getting the like $50-80 Mechbox later. The mysterious sound however i will investigate when i tear open my mechbox again to install the new spring.
One quick question that isnt really related, how strong are the tabs on the plastic body? Its a pain to put the bodies together now since i got a G&P metal hop up. Im afraid to break off the tabs... it would give me a nice excuse to get a metal body

[Omi-san]

The new type TM M4A1 is supposed to have reinforced tabs. As long as you make sure they are going in the lower receiver when closing the upper, it should be fine.

How does the metal hop-up fit? You remove the plastic hop-up feeding tube from the front of the mechbox and that's it?

[sgtscarfy]

To fit the one piece metal hopup isn't too bad. You have to take a dremel tool and eat away a bit at the top of the mag-well. After this mod/upgrade you cant breakdown the m4 the same way anymore. You have to put the lower and upper together on the same plane(as in both horizontal orientation). It difficult to put the plastic body together with the mechbox nozzle in the hopup and at the same time put both bodies on the mechbox all the while trying not to break off the tabs. After about 10 min of messing with it I finally got it together...
Apparently a metal body will solve this problem but its expensive...
Is it worth it? I dont know? All I know is im gonna have to do it again when i put the different spring in the mechbox...

[ILLusion]

That sound is your spring and gearbox resonating from the heavy and rigid impact of your aluminum piston head against your gearbox.

Expect your gearbox to break soon, if you keep using that piston head on your version 2 gearbox. Some users have broken their gearbox when using metal piston heads with a BONE STOCK SPRING.