V2 mehcbox piston problems.

[Flatlander]

Well a new guy here just recently purchased a slightly used upgraded CA M4 and within approx. 100 rounds it locks up on him.

Important background on the internals:

-Prom. SP120 spring
-Guarder bearing piston head (polycarb or AL, unknown)
-Deep fire metal nozzle
-Systema bearing spring guide
-*Possible* systema RED piston
-Gun and all the parts very new and most likely less than 3000rnds

So while firing the gun jams. He clears the jam with the cleaning/unjamming tool. Fires some more and it jams again and the mechbox locks up. He disassembles it himself a bit and is able to look into the cylinder port hole and sees red chucks inside as well as the part of the RED piston in the rearward position (ie. locked up w/ spring compressed).

It was not mentioned if the piston was also upgraded and as far as I know only systema makes a red piston, correct? And I've read these should be avoided at all costs, correct?

Also, it wasn't mentioned if the piston head was aluminum or polycarbonate. I was able to find a guarder bearing piston head in aluminum and am not sure if a polycarb exists? I know aluminum heads should NOT be used in a V2.

Anyways, I'm not completely aware of the extent of the damage as I have not yet personally seen it. He's probably going to be bringing it around for me to have a look at pretty soon but from talking with him it sounds as if the front of the piston exploded and then locked up the mechbox. So, does anyone have any ideas of WHY this happened?

-Spring to heavy?
-Poor parts choice?
-Improper installment?
-Combination of the above?
-Did the jamming have anything to do with the failure?
-Other?

This guys is brand new into airsoft and this was his first gun so I'm hoping this can be solved and is a relatively easy and cheap fix! I'm no gundoc so hints on what I should be looking for or other possible damaged areas. I don't plan to fix it for him unless it's a simple piston replacement.

Also, what type of piston should he get? I'll probably convince him to go down to a sp110 spring. I've heard good things about stock TM pistons. The piston head may also have to be replaced; suggestions?

Thanks guys.

EDIT: I'll update after I've opened up the mechbox.

[zone 69]

Maybe loctite was not used on the piston head and it came lose and fell off. I heard of this before. For new part,s i went with a systema silent piston head set in my v2 mech box,,it will be easyer on your mech box because they have a rubber bumper on the piston head & the cylinder head. For the piston itself i don't really know what,s best. O and Guarder dose make a red polycarb piston head.

[six4]

Originally Posted by Flatlander

Well a new guy here just recently purchased a slightly used upgraded CA M4 and within approx. 100 rounds it locks up on him.

Important background on the internals:

-Prom. SP120 spring
-Guarder bearing piston head (polycarb or AL, unknown)
-Deep fire metal nozzle
-Systema bearing spring guide
-*Possible* systema RED piston
-Gun and all the parts very new and most likely less than 3000rnds

So while firing the gun jams. He clears the jam with the cleaning/unjamming tool. Fires some more and it jams again and the mechbox locks up. He disassembles it himself a bit and is able to look into the cylinder port hole and sees red chucks inside as well as the part of the RED piston in the rearward position (ie. locked up w/ spring compressed).

It was not mentioned if the piston was also upgraded and as far as I know only systema makes a red piston, correct? And I've read these should be avoided at all costs, correct?

Also, it wasn't mentioned if the piston head was aluminum or polycarbonate. I was able to find a guarder bearing piston head in aluminum and am not sure if a polycarb exists? I know aluminum heads should NOT be used in a V2.

Anyways, I'm not completely aware of the extent of the damage as I have not yet personally seen it. He's probably going to be bringing it around for me to have a look at pretty soon but from talking with him it sounds as if the front of the piston exploded and then locked up the mechbox. So, does anyone have any ideas of WHY this happened?

-Spring to heavy?
-Poor parts choice?
-Improper installment?
-Combination of the above?
-Did the jamming have anything to do with the failure?
-Other?

This guys is brand new into airsoft and this was his first gun so I'm hoping this can be solved and is a relatively easy and cheap fix! I'm no gundoc so hints on what I should be looking for or other possible damaged areas. I don't plan to fix it for him unless it's a simple piston replacement.

Also, what type of piston should he get? I'll probably convince him to go down to a sp110 spring. I've heard good things about stock TM pistons. The piston head may also have to be replaced; suggestions?

Thanks guys.

EDIT: I'll update after I've opened up the mechbox.

www.mechbox.com will help you with the tear down/rebuild.

[Flatlander]

Thanks six4, been there a few times already. I'm somewhat competant on the teardown/rebuild. I've currently got my CA m4 completely apart awaiting for a few more upgrade parts to arive. I found my airleak (cylinder bore was too large and wasn't sealing properly - systema cylinder seals fine).

Good call on the loctite zone. I'll check for residue left on the threads.

[zone 69]

I allso read some where that some spring,s jam up on the systema spring guide,s. I think they were Guarder spring,s im not sher.

[Skruface]

I'm gonna sugest that having both a bearing piston head and a bearing spring guide may be the problem. I had a similar problem years ago with an MC51 - the mechbox would lock up full to the rear for no apparent reason.

As I figured it out, the spring (Angel) was too long when used in combination with bearings at both ends (piston head + spring guide) and would be fully compressed before the piston was all the way back, locking the piston/gears.

Here's a quick-and-dirty suggestion: remove the piston head bearing and replace with a washer and shorter screw. See if the gun will cycle that way. Alternately, replace the piston head with one that doesn't have a bearing.

As an aside, removing the extra weight of the bearings will also lighten the piston assembly. The reduction in reciprocal mass should give you higher rate of fire.

[mcguyver]

I'm leaning towards the pring being too heavy. In the past when I've used Prometheus springs, I found them to be about 10% under-rated. For example, the MS110 gave a velocity of 400 fps, MS100 was 365 fps, MS90 was 330 fps, etc. I would expect the MS120 to be about 430 fps. With a standard ratio gear set, That's a tough load, and my ICS M4 used to lock up when I had the MS110 in it, even with a big battery.

The "SP" designation you used is Guarder, not Prometheus.

[CDN_Stalker]

I have an MP5 with the only velocity upgrade being a Prometheus 110 spring (stock spring guide, 229mm barrel, etc.), and I get pretty consistant 365fps readings from it. Which is pretty decent considering the m/s rating to fps is about 361fps with 0.20g BBs. So it'd be higher with a longer barrel. BUT, I have a used Prometheus 120 spring I haven't installed yet, and apparently it put out 370fps in a TM M-14, so I don't know what's up with it.

[Flatlander]

**Gun Update** Opened it up myself today

- Polycarb piston head pretty much exploded. It did not fall off the piston.

- Has a green polycarb piston (deepfire?)

- Wrecked the cylinder head.

- Gears are in excellent shape. Axles do not appear to be bent that I can tell (quick 'between the fingers and spin test' - know of another way please let me know!)

- Piston appears to be okay EXCEPT the second last tooth (or second in from the back of the mechbox) is very warn. It is not worn down but rather almost worn in half somehow. Should I replace this piston? I believe I've read that this tooth can be filled down completely and will function fine (and is good practice on high ROF guns)...or do I have the wrong tooth in mind?

Anyways, so what I need some help with is:

- What is a good quality polycarb piston head and whatever type of cylinder head.

- Should I replace the piston or just file the gear down? If so, will a stock TM one suffice or would you recommend another brand? NOTE: he insists on using a MS120 spring but I convinced him to go lower for the few winter games.

- I'd imagine the gears should be taken out, cleaned, reshimed, greased to remove any foreign piston head particles?

Appreciate the help guys!

**EDIT** Forgot to mention that he already has a bearing spring guide so no real reason to have a bearing piston head (he's a newbie and doesn't really need the little extra FPS right away)

[Flatlander]

Bump...I'd appreciate to hear what gundocs would recommend from my last post above, thanks.

[CDN_Stalker]

Replace the cylinder head (not overly sure of the damage or type though), piston head, piston body and try again.

[zone 69]

I went with a systema silent piston head set in my v2 mech box,,it will be easyer on your mech box because they have a rubber bumper on the piston head & the cylinder head. For the piston body itself i don't really know what,s best. I like to know this info myself.

[joaz]

Originally Posted by Flatlander

- Piston appears to be okay EXCEPT the second last tooth (or second in from the back of the mechbox) is very warn. It is not worn down but rather almost worn in half somehow. Should I replace this piston? I believe I've read that this tooth can be filled down completely and will function fine (and is good practice on high ROF guns)...or do I have the wrong tooth in mind?

That's the result of an installation error. The sector gear keeps touching the 2nd tooth when it should engage the first tooth (I always count from first tooth = big one that sector gear first catches). If the piston sits too far backward, you need to cut the second tooth down or remove it completely.

The piston should be fine to re-use.

On the other parts... well if you get a plastic piston head, get a guarder cylinder head. They have a flat back. On downside they are very heavy, so you might going better with a light cylinder head and metal piston head.

Tapered cylinder heads will break your plastic piston heads in no time.

On plastic pistonheads avoid red guarder and yellow CA. I would recommend Systema on this. Metal pistonheads rarely break at all.

[zone 69]

It,s away,s a good idea to match your piston head with your cylinder head.
So if you buy a systema piston head you get the systema cylinder head to go with it.
Same with Guarder piston head you get the cylinder head to go with it.
Unless you know the brand,s you are mixing will match up.

[eel one]

Not necessary to match brands, you will be fine mixing them. The systema silent piston head works fine with the laylax/prometheus cylinder just to give one example. The important thing is that they fit together.

As to the gear ratio. Using a M120 spring on ordinary gearing is common practice here in Denmark. Just make sure that you have done a proper shim job so the gears guns smooth and use a battery on proper cells (I'll say minimum GP2200 4/5 subC, but the ELITE4500 should be the bomb). The stock EG1000 should not have a problem turning the gears with a reasonable ROF. But you will most likely need to change the fuse to a 25 amp.

The piston is fine. And yes it is most likely a Deepfire. I have found these pistons to be very similar to the TM stock piston which is a very fine piston if you don't go above M120. Just trim away the second tooth, it will also give the gun a little buffer in the event of a jammed BB in the barrel and may prevent a false engagement between the sector gear and the piston.

As the gun have a v.2 gearbox, DO NOT use a metal piston head. It will cause the gearbox to crack prematurely especially in winter temperatures. From that same reason I would go with a systema silent head set. It will soften the blow on the gearbox cylinder window where the v.2 box usually fails.

As it is a used gun, a re-shim could be a good idea. I always open the gearbox and give it a service check when a new/used gun gets to me. Manufacturers does it to quickly and I don't know the mechanic skills of a previous owner.