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Hi! So my friend owns an m14 AEG electric automatic gun which makes grinding sounds randomly. I pretty much replaced everything in that gearbox and shimmed it which I have done too many times to count. Gears are steel SHS regular ratio gears which make average amount of noise. Everything else is also from SHS and spring is m120 (1.5j). The gearbox had a little flaw which caused piston to get stuck when fully loaded and the sector gear was too fast and engaged the piston too soon before it was fully fired and broke 2 pistons. Problem was those middle rails that were too high, not anymore. This was the reason that I opened the gearbox in the first place. The piston has just enough play to move freely so it can't get of the gears to make that grind sound... So now the problem is that gun makes grinding noise randomly when it's fired with BB and not without. I have tested the gears separately including jamming them from sector gear and of course this causes gears not to move which means that the gears are not flying over each other and there are no grinding sound. I have also checked that motor is engaged correctly (original). 11.1v Li-Po 20C is used. This exactly the same configuration that I use in my m4, same parts from SHS, of course gearbox exclusive parts are different, no problems there. First I suspected that the grinding sound was caused by pistons full width teeth to grind on bevel gear and pretty much machined that piston to be half teeth piston (full metal teeth, first one cut of for AOE correction + sorbothane is used). The sound is still there when shooting with BB and the piston is not hitting any gear but sector as it should be. So now I checked that piston hits the sector gear correctly and there is nothing but normal teeth engagement. The gears are black so if there would be any grinding we would see it on the parts, but of course nothing. Piston moves freely in the cylinder when the gearbox is fully closed with screws and gears too of course. The gearbox is straight and there is nothing that would indicate that gearbox is the problem including no grinding marks... Anti-reversal latch is working correctly and cut of lever isn't the problem. So my next guess is that piston does not make it all the way to the end of the cylinder before bevel gear makes it's next revolutions, but there is nothing to stop that airflow or atleast I don't see it. So it seems like the piston getting somehow stuck is still the problem... Only thing that restricts that airflow is BBs them selves but the gun still shoots normally... So could the hop-up rubber be the problem? The hop-up chamber is quite big and even if the nozzle (metal+o-ring) is tight fit the air should still get out... I don't see nozzle and nozzle latch make any difference in making that grinding sound. There are less parts in version 7 gearbox then 2 or 3 and that makes fewer brake points... So as you can see I pretty much have a gun that is working correctly but makes random grinding sounds that are as loud as motor not hitting spur gear correctly... Nothing is broken nor there is any wear... I have done everything I can, but still the problem occurs and it's somehow related to piston. Please help me figure this out!