http://www.pbnation.com/showthread.php?t=3724216
015 Urethane 70 durometer o-rings. Standard tank o-rings are the correct size, but wrong material and harness and do not work.
In a pinch (with a more difficult removal later)
90 durometer buna o-rings for this. The extra hardness seems to be just enough to safely slip them past the obstacles that are faced in this operation.
