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Opening the Mechbox for the first time...I'm scared!



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Old December 3rd, 2012, 08:38   #1
Join Date: Mar 2012
Location: Picton, Ontario
Opening the Mechbox for the first time...I'm scared!

Hey guys,

lots of you have been very helpful answering my questions of late. Thank you. I have more though...

I have a G&P M4 MRP. Been using it all season (10 000 + rounds through it). I have had no problems, but I know it should be maintained every season and all that. So...

Do you guys have any tips/suggestions/videos that I should be aware of before breaking into the box???

Currently, my M4 has a (apparently) M120 motor, a metal piston and head, metal spring guide (no bearings), a 6.03 Barrel from TO Airsoft, shoots about 415 FPS with .2's and has no mods or maintenance besides this.
-What things would you guys suggest I do to this lil beastie? I was thinking an SHS piston and a Sorbo Pad, a lighter spring (get me under 400FPS).

Thanks for your help.

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Old December 3rd, 2012, 09:00   #2
venture's Avatar
Join Date: Jun 2005
Location: Halifax
While it is good to do maintenance on your gearbox, I am a believer in not fixing items that aren't broken. Maybe you can go another season without touching that gb.

If you are interested in learning the job, then take it apart. If you are just concerned about maintenance, don't worry until you have 40 or 50 thousand shots through it. Then you can inspect it for wear.

My TM MP5K PDW had over 100, 000 cycles through it by the time I opened it up the first time. It was like new inside.

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Old December 3rd, 2012, 09:09   #3
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Join Date: Apr 2006
Location: Toronto used to be a great source for How-To videos. They did a "replace your spring" guide for a TM M4. While the disassembly of the rifle is different from yours the mechbox is the same. Don't know if they're still around.

There's lots of other Youtube videos as well...I think Airsoft GI did some decent ones and there's probably hundreds of homemade video ones as well.

Watch a bunch of those...then dive in. Some general tips...
- have a little tray or a bunch of cups to put screws and parts into (i.e. the grip and the grip screws and the grip plate...etc...)
- have a little bag to put your motor into (keeps debris from getting into it)
- use allen keys and screw drivers that fit properly...stripping a little screw is a PITA to get out
- work on a'll help with preventing little springs and screws from rolling off onto the floor
- if you feel like you need to...put a towel over the mechbox when you're opening it...that way if it gets away from you all the bits won't be flicked across the room
- if you get it apart without any problems (i.e. the gears pop out and you're looking at a bunch of shims laying randomly about)...then just take a look at how each piece works, how things work, reassemble it and test it. Then take it apart to clean it up and make changes
- there's probably shims (thin metal discs) on both the top and the bottom of the gears on the axel. Sometimes they're stuck to the gears...sometimes they're stuck to the bearings/bushings. Just pick them off and keep them together.
- it's easier/clearest to re-shim the gears if there's nothing but the bushings/bearings in it
- you don't have to put the whole thing back together to can just put the mechbox on the grip, put in the motor and manipulate the side selector plate for safe/semi/full.

You're going to want to consider having the following on hand:
- allen key set
- rubber safe grease
- paper towel or rags
- regular sized and micro + screw drivers
- needle nose pliers
- shim set (a package that has lots of various shims thicknesses)

For upgrades...I wouldn't advise a first timer doing any on the first go when they're doing it themselves (unless something's broken). Just take it apart...clean the parts...reshim/regrease...reassemble/test. That way you're pretty sure you've got everything in order. You'll want to take a good look at the fins of the switch assembly. They'll probably look burnt/corroded. You can touch them up with a bit of fine emery cloth....if there seems to be enough metal left then you can leave it at that, if not, replace the switch. Then...unless parts are overly worn just change the spring. If after 10K your piston is in good order and the teeth of your gears are ok, then count yourself luck and leave the rest of the mechbox alone. A M110 will get you under a bunch of test mags through the gun before you chrony.

You'll want to consider changing the hopup rubber while you've got it apart...and really clean the inner barrel if you do that. A cloth patch with silicone oil does the trick...or windex. Don't use any polish/abrasive/cleaners.

If you've got a bit of extra a second mechbox. Keep one working at all times while you monkey around with the other. That way you won't find yourself in a jam with a game coming up soon and your mechbox a mess on your bench.

Best of luck

***Venture makes an excellent point***
- if it's been solid all year...then just leaving it is an option for sure. TM guns (which used to run at 280-300fps vs. 400+) would go for ages with almost no wear. The metal contacts on the switch or the motor would wear out well before the internals would (generalization).
- but it's still not a bad idea to take your hopup apart, inspect your houp rubber (for wear/tears) and clean your barrel.

Last edited by m102404; December 3rd, 2012 at 09:12..
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Old December 3rd, 2012, 09:46   #4
a.k.a. Greedy
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Join Date: Dec 2010
Location: Toronto
Nothing wrong with the, "if it ain't broke" approach but if you chose to make performance upgrades then now is a good time to do it if you have it apart already. As far as what upgrades to make it really depends on what aspects you want to improve.

Most people are concerned with range and accuracy. For this you want to improve your compression and your hop-up and hop-up rubber. Do a compression check and see where you could be leaking. Does your cylinder head have a double O ring? Does your nozzle have an O ring? These are things that may be worth upgrading. You might also want to try a new style hop system. One of the new flat or bridged style nubs that are on the market now, maybe a shredders type nub. Alternatively you could stick with your traditional style nub and get a firefly or PDI W-hold style rubber.

You should get your gun below 400 FPS as that is the common max allowed for outdoor games. No guarantees but an M110 should do the trick. I like to put bearing spring guides in mine it makes them run smoother and gives me better battery life. A re-shim is usually always a good idea it has many benefits.

Your motor is fine to me with an M110 spring unless you want to increase ROF, but there are other ways of doing that too. Unless your piston shows signs of wear it's should be fine unless you increase ROF in which case you may want to switch to a full metal rack.

Sorbo pads are a nice addition and will help you correct your AOE. Just be aware it's not as simple as just dropping the pads in, you will probably need some custom modification such as shaving down a tooth on your piston.

It all really depends on how far you want to go with it. What I would recommend for you being as it's your first time opening a mechbox is: Clean and re-lube it, Check your compression and correct any poor seals, re-shim and a PDI hop rubber, making sure the rubber is lined up straight when you install it. Other than that just make sure you remember how everything goes together and don't lose anything. Keep your stock parts around because if something decides it doesn't want to work with your set-up you can always go back to the last known working configuration.

Last edited by Kozzie; December 3rd, 2012 at 11:09..
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Old December 3rd, 2012, 10:10   #5
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Join Date: Jun 2005
Location: Halifax
Good advice in both of the above posts. My suggestions was simply a practical piece of advice, but as I said, if you want to learn then look carefully at the above 2 posts. When I was new at this, I would take a piece of paper and draw a rough gearbox shape, then poke holes where the screw holes on your gearbox are located, then put the screws from the gearbox in the appropriate holes in my template. Then you know you are putting the correct length screw in the holes when reassembling.

-- Example: Put a motor mount screw up where the selector plate is and watch the fun as the plate tries to slide over the 1mm of screw that is protruding through the gearbox.

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