Threaded vs. Universal Winter Pool Plugs: What I Use and Why

Hey everyone, Al here! In the last post, we talked about warm fall water under a closed pool. Now let’s focus on the small parts keeping that water out of the plumbing. Winter plugs are inexpensive compared with a cracked line, but they are not interchangeable. The two styles I reach for most—and our team regularly works with—are threaded plugs with O-rings and universal expansion-style plugs.

Threaded Plugs with O-Rings

A threaded plug screws into a matching female fitting. The O-ring creates the seal at the face, so there is no need to crank the plug down with excessive force. This style works well when the return or fitting has clean, undamaged threads and the exact size is known.

The CMP 1.5-inch raised threaded plug with O-ring is a good example. Its raised head is easy to grip at closing and opening. Before installing one, clean the threads, inspect the O-ring, and replace any seal that is flattened, cracked, or missing.

Universal Expansion-Style Plugs

An expansion plug seals by compressing a rubber body inside an opening. This style suits a smooth pipe opening or a fitting designed for that plug’s size range. It is also useful when damaged threads prevent a dependable threaded seal, provided the inside surface is sound.

We carry the Technical Products SP710 universal 7-to-10 plug. “Universal” does not mean it fits every opening on every pool. Always measure the inside diameter and follow the stated fit range.

Which One Fits the Job?

• Clean matching threads: A threaded O-ring plug provides a positive, repeatable fit.
• Smooth or unthreaded opening: Use a correctly sized expansion plug.
• Cracked fitting or rough sealing surface: Repair the problem instead of expecting a plug to hide it.
• Unknown size: Measure first and order second.

Whichever style is used, install it while the line is being blown so air stays trapped in the plumbing. Tighten it only enough to seal, then check that it has not shifted before the first hard freeze. I keep a simple note of each plug’s location and size because it makes next year’s closing much faster.

You can compare additional sizes in our winter supplies collection. Next time, we’ll move back above the waterline and decide when standing water on a solid cover needs to be pumped—and when it is better to wait.