Its a .015 size oring. It mentions it in the trouble shooting part of the manual.
"If the leak comes from the small hole in the middle
of the regulator there are two possible O-rings
causing the problem. Change the #015 O-ring on
the piston and the #007 urethane O-ring inside
the body of the regulator."
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Angel A1 magno valved f/s/t
lightly upped sft shocker $200, eigen bolt $55, Virtue redefined board $70... the look on an eclipse agglets face when they realize their brand new $1250 gun just got out shot by an 03 spoolvalve PRICELESS