The schematic identifies it as:
O ring φ2.4*φ1.8 hardness 90° JSR |
The 1.8mm figure translates to 0.070in which is usually a reliable indication it is in -00x series of SAE O-ring sizes. However the 2.4mm figure (0.094in) does not match up to any of the standard sizes, and is instead somewhere between a -004 and -005:
View attachment 417175 If this callout is correct, someone at Air Venturi deserves 20 lashes with a wet noodle, a clogged bathtub drain, and an infestation of bed bugs. Not so much because it is a nonstandard size...a supplier like theoringstore has lots of nonstandard sizes available. But it is a nonstandard size in 90 durometer, which dramatically diminishes the odds of finding a match.
So I hope the callout has a typo and someone will come along to correct it.
Or do you have a set of calipers to measure the thickness and OD? EDIT: Your closeup photo is clear enough to tell me you have the correct size installed. It fills the groove and the apex stands just proud of the piston. Meaning the source of the leak is likely a different O-ring, or perhaps a scratch or other defect in a mating surface.
A copy of the schematic is attached for reference.