Oh No! O-Ring: The Truth of Not-So-Simple Leaks

I wish the statement, “Oh, it’s just a blown O-ring,” was a simple as it sounds.

Sometimes it IS simply a matter of matching the damaged O-ring to a new O-ring from the box of assorted SAE O-rings you bought at the local auto parts store. Problem solved. But what if you can’t find an exact match in the SAE box, or the original O-ring is a weird color or has flat sides?

That’s when you enter the world of SAE vs. metric O-rings, special compositions and special designs. A world where metric O-rings are fractionally smaller in cross section than SAE O-rings, where the difference between Nitrile, Viton and Buna-N is the difference between leak/no leak, and where O-rings with round, square and x-shaped cross-sections are not interchangeable.

In general:

• SAE O-rings and metric O-rings are generally not interchangeable. Using a small-diameter metric O-ring in the oversize groove of a SAE fitting often causes leaks because the smaller O-ring doesn’t compress properly. Similarly, an oversize SAE O-ring in a smaller grooved metric fitting is so big that it may get cut or crimped when the fitting is tightened.
• O-rings that fit in a groove AROUND a fitting are generally SAE. O-rings that fit in a groove in the flat-faced END of a hydraulic fitting are generally metric. They’re often sold specifically as “Flat Face O-rings.”
• If an O-ring is brown, white or a color other than black, it may be a special composition designed to withstand exposure to unique chemicals or designed for higher or lower than normal pressure ranges. Consider those colors warning that a special O-ring is required. 
• O-rings with square cross-sections are often used in unique applications. O-rings with an X-cross section are a variation on this design. 

And finally…the softness or hardness of O-rings is quantified according to their Durometer rating. The lower the Durometer rating, the softer the o-ring:

• Duro 20A = the consistency of a rubber band.
• Duro 40A = a pencil eraser.
• Duro 60A = car tire tread.
• Duro 70A = a running shoe sole.
• Duro 80A = a leather belt
• Duro 100A = shopping cart wheel

For most agricultural applications, Duro 70A O-rings are the norm.

Tool of the Trade

Yes, it’s possible to use a small screwdriver or a piece of baling wire to remove rubber O-rings from hydraulic fittings or valves. But a set of $10 to $30 O-ring picks makes removal and installation much easier. 

Traditional O-ring picks look like the tools your dental hygienist uses to clean your teeth. A new breed of O-ring picks is now on the market with tiny rounded and flattened ends rather than needle points. Needle-point picks are great when you have to snag and remove a damaged O-ring, but the new flat, blunt picks are handy when it’s necessary to nudge a stubborn O-ring into its groove, or if you’re planning to re-use the O-ring and don’t want to prick or nick it. 

Avoid picks with screw-on handles — you’ll be forever annoyed because at the very moment you need to apply circular leverage, the #&%! handle will unscrew and derail the delicate maneuver.