check your tappet plate and nozzle.

I was looking in mech box and couldn't find tappet,what does it look like?.

it looks like this
http://www.wgcshop.com/WGC_Shop/images/sys_a13.jpg

Thanks. I had that out and was looking for cracks on it,none.
The spring pushes it forward quite well and it moves freely.
The noise i was hearing was the partial gear stripping the
plastic tube attached to the piston.I am not sure if it is a original
TM part,I had a spare and in stalled it,I thought I would temp fate
and put the sp110 guarder spring back in.I put the pistol grip and
motor on the mech box and everything went fine.I placed the V2
in the body and it still work great,now I put a empty mag in still works,
I put rounds in and it cycles put nothing come out,after three cycles
with the rounds in the noise came back and now the piston assembly
is at the rear as the moter will not finish the cycle.This will be the 8th
time I have taken this apart and I still can't fathom what the hell is wrong.

Well the good news is that I am getting good at taking this apart
and reassembling it.Here is something ,it works normal running it with
the mechbox and motor,then I place it in the upper bodyusing the
rear lock pin secure it for my next test.Next I hold the loaded mag
in position and you hear the round load into the chamber,on semi I
pull the trigger and it sounds good put no rounds come out,I pull
the trigger again and you can hear a grind noise,I take the box out
and the tappet and nozzle are compressed to the rear but the piston
is ahead.Now I have checked the shims and the movement of all parts.
PLEASE HELP LOL , what causes this to get messed up?.

Don't know if this will help you or not, but I found this video extremely helpful when it came to disassembling and reassembling my TM M4A1's mechbox...

http://www.mechbox.com/site/mechbox-...ade-video.html

A big THANKS to you guys for your help,I did watch the vid on mechbox,very helpful.Now to tell you what the real problem was,the hopup was not letting
the BB's through,I went to clear the barrel from the breach and could not push
the BB through.I took the hopup off and this little tiny tiny rubber tube fell out,
I tried things back together and it worked.I reset the timing on the gears upgraded the spring back to the sp110,reassembled,and it worked.Near as I
can tell is that the BB was creating a back pressure causing the piston to float
that when the gear came around again to cycle, the piston was in the wrong position and would grind due to off timing.Boy did I learn alot this weekend,
we can call this case closed.:cool:

I thought I would put one last thing up.I tried this popcan test I have
read about here,at two inchs it goes through both sides,on the bottem
it makes a hole but can't finish the trip inside.I used a .2g BB sp110
spring and a alunimum coke can.Would this translate to about 380-400fps?

that should be about right.

as for your hopup sleeve, i was just about to say something about that, but its good that you figured it out on your own.
i had the same issue with my prowin 8mm mechbox. i stripped 2 pistons befor i figured out that the hop sleeve was out of spec, and not letting the BBs get pushed into the hop by the tappetplate.

and easy test for this - remove the barrel and hold it with the hop sleeve pointing up, place a BB on the oppening to the hopup, then push it through.
the oppening should be just narrow enough to lighty hold the BB from falling though and it should drop in with a light push of a finger.

Well, DT, it's nice that you're learning. I know I learned a lot about my gun and mechbox over the past week. I've disassembled and reassembled my mechbox about 10 times now following the video I posted. I don't know for sure if the gun will fire once reassembled, because I have't received my replacement parts yet, but I think I should do ok.

It's nice to have such great support from experienced members here to guide us when we get into touble with our guns.

sorry for the Thread jack but i have a gearing issue i bought a Guarder 120 tuneup kit and have tried the gears (they are original type steel gears) but everytime i install them they lock up with the spring in the fully compressed position tried them in 2 seperate guns and the same thing happens pulls the spring all the way back and jams.Is it possible the gears are warped or mislabled as something else as soon as i put the other gears back in it works great have even tried them without any shims at all ending with the same result please any idea's would be appreciated oh yeah and its not the battery because ive tried 4 different batts

when you pull the trigger, does the gears stop suddenly or do they seem to whine down and stop?

it could be that the gears were miss labeled, and you got a highspeed gear set, when you need to be using at least a torque up set.

they stop suddenly and when i take the mechbox apart there doesnt seem to be enuff room for it to come back any further but yet there is still 1 tooth in the piston to go through

make sure that the piston head is seated properly, and that the barings/bushing on both the inside of the piston and the back on the spring guide are seated properly.
if not, the spring may reach full compression befor the piston comes off the sector gear.
that is the only thing i can think of that would cause your piston from fully moving back.
if you have installed an extra bushing behind the piston head, it would cause this to happen.

the thing is ..is as soon as i change the gears back to the originals it works flawlessly attempted those gears at least 10 times now and they dont seem to wanna work im gonna go with mislabled and be done with them.I would try to sell em but not knowing what they are for sure i wouldnt want to mislead anybody oh well