How is your sector tooth to piston tooth alignment? Is the first sector tooth grabbing the piston at an angle? If not, then is the first sector tooth contacting the 2nd piston tooth?
Both of these are very common issues, and responsible for many a stripped piston.
Pictures are worth lots of words, so view this thread for more info:
http://forums.airsoftmechanics.com/i...hp?topic=956.0
The solutions to both of these problems are simple. First, to get the 1st sector and piston teeth to engage at a better angle, you can either add some washers to the piston head, or add some sorbothane to the cylinder head. Either way you are pushing the piston back a little causing the sector gear teeth to engage with more contact area. I prefer to add padding to the cylinder head, which prolongs the life of the mechbox and does not slow down the piston (as adding weight in the form of washers, does). If you use sorbothane, you can purchase it at
www.mcmaster.com for a whopping $4.47 US... i recommend the 70-OO hardness.
As is often the result of doing this, however, the first sector tooth will want to grab the 2nd piston tooth. The solution there is to grind down about 1/2 of the 2nd piston tooth with a dremel.
I did both of these things to my friend's G36, and its piston looks brand new after a year and 50,000 BBs through it on a PDI 190% spring (shooting around 400 FPS on full auto).