This is a basic piston i printed out to test the pneumatic capabilities of printed objects. I set the fill up to .7 but still have some holes in prints. These holes vary in between a couple of cracks to many holes. In the future i'll be designing thicker walls but if anybody has any ideas and/or comments you're welcome to share them. The piston cap is designed slightly bigger than the piston cylinder walls so that it can be filed down to a perfect fit with the piston walls

File down the piston cap until it fits perfectly with the cylinder walls. Play with the fill ratio. I got mixed results between .2 and .7 on skeinforge