Hi @Zissou,
Its true the o-ring sizes we use do not quite match the nominal recommendations of the Parker O-ring Handbook, we use a size slightly smaller on all our enclosures radial seals.
Normally, o-rings should be installed with minimal stretch, but In larger piston bore sizes this can result in the o-ring sagging out of the groove, making installation challenging and pinching likely.
In this case, going down slightly in size is permissible to eliminate sag in order to ease installation and reduce the chances of damage.
You can find this information on page 3-8 of the Parker O-Ring Handbook.
We’ve thoroughly tested our enclosure o-ring seals in our hydrostatic pressure chamber, and are confident in their performance.
-Adam