Pentair Waterfall/Challenger Pump Diffuser O-Ring, 355331Z

The Pentair 355331Z is the diffuser o-ring for the Waterfall and Challenger pumps. It is the genuine Pentair part, cut and sized to the original.

What the diffuser o-ring does

The diffuser sits over the impeller and turns the impeller output into usable pressure. For that to work, the diffuser has to be sealed against the housing so that water is driven through it rather than allowed to slip back around the outside. The o-ring is the piece that makes that seal. It is small, it is inexpensive, and when it goes flat or cracks the pump quietly loses flow with no obvious cause. Owners usually chase the basket, the skimmer and the impeller first.

When to change it

Any time the pump is open. The diffuser comes off for shaft seal replacement, impeller changes and blockage clearing, and the o-ring is disturbed every time. A compressed o-ring that has taken the shape of its groove will not reliably reseal once it has been moved, so it is worth replacing rather than reusing. Seat it clean, use a silicone based o-ring lubricant, and check it has not rolled in the groove before the diffuser goes back down.