I have a version 3 gearbox, not sure on the make, I have just repaired it ((bearings) and upgraded the spring at the same time. Trouble is when I pull the trigger it pulls back the piston almost all the way and then stops. Ive taken the spring out and tried it without, the gears all rotate fine and pulls the piston back all the way.
This my first time working on a gearbox so any pointers would be appreciated 
Could it be anything to do with the timing? both the bevel and sector gears have an X stamped on them. I have aligned the sector gear in what seems to be the optimum position so that the tappet plate fits comfortably which positions the X facing straight up. 
Sounds like your new spring is too long! It's compressed as far as it can, which still isn't enough for the mechanism to cock fully back. Trim a few coils off of it.
A Proud Member Of 'Team Spleen!' who play mainly at Gunman Airsoft, Tuddenham, Suffolk.
I thought the same at one stage so put the old spring back in, but it did the same thing 
Have you added a ball-race set to your spring guide?
Or is your spring guide being seated back in the housing far enough?
Your checking it out with a good full battery, I'm presuming?
A Proud Member Of 'Team Spleen!' who play mainly at Gunman Airsoft, Tuddenham, Suffolk.
Have you added a ball-race set to your spring guide?
Or is your spring guide being seated back in the housing far enough?
Your checking it out with a good full battery, I'm presuming?
1) No
2) will check tomorrow
3) yes all charged
so all that's been changed is bearings and spring and you try'd the old spring and are sure the battery is charged (flat battery's catch us all out some times).
you may have over shimmed it causing the gears to be tight,easy to test,strip the gearbox and remove the spring ,piston tappet plate and cylinder so you can see the gears,then close the gearbox and put a screw or two in and the top slide on clip thing,do the gears turn freely,are they tight or rubbing against each other.the objective with shimming is to have the maximum area of teeth in contact with each gear and to minimise left to right movement,there should always be a little left to right movement but it should ideally be as small as poss .also as to timing ,when assembling the sector gear(the one that has the half set of teeth for the piston) should have its cam (the small rod on its side that pulls the tappet plate back) at the 2 o'clock position and the bevel gear(the one that is turned by the motor) should be locked back on the anti reverse latch.
hope that made sense and helps,shimming is fiddly and you cant assume that when you change bearings that it wont need re-shimming because the bearings are slightly different thickness,on the bright side shims are cheep as chips from any airsoft shop.
Dadio
armoury
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I agree. Over shimming is the most common cause of iatrogenic (sorry, doctor speak for "of your own making") gearbox failure. Try reshimming the gears one by one starting with the bevel gear relative to the motor. When you check if the the gears spin freely, always close the gearbox and screw it closed.
