That box of assorted rubber O-rings under the workbench in your shop might supply the cure to leaky hydraulic connections, or it might just interrupt the leak for a day or two.
Flat-face hydraulic fittings and connectors have an oil ring in a groove in the face of the male fitting. That O-ring seals against the flat, machined face in the corresponding female fitting.
O-Ring Boss (ORB) fittings are the “standard” hydraulic fittings we grew up with, where a threaded male fitting has an O-ring at the base of the threads. The O-ring seals against a chamfer machined into the matching female fitting.
Flat-face and ORB fittings look similar and might fit on either style of fitting, but they won’t work well on the wrong fitting.
There are detailed engineering specifications detailing not only the precise size and cross section of the two distinct types of O-ring, but also for the different grades of elastomers and rubber used in specific applications.
The easy-to-remember bottom line is that an O-ring used in a flat-face hydraulic fitting needs to come from a box or bag labeled “Metric” or “Flat-Face” O-rings.
An O-ring for an ORB hydraulic fitting must come from a container labeled “Standard” or “ORB” O-rings.
If the box or bag is illegible from spending the past year in the bottom of your tool box, you’ll know in a day or two, maybe mere minutes, if it’s the wrong type of O-ring.
