Open-end vs threaded hot tub filters: how to tell which fitting you have

Last reviewed October 27, 2026

Match the top and the bottom, not just the outside size. Unicel’s and Pleatco’s spec tables print an opening size, a handle, a male slip or a thread for each end^[5][2]. A 7" × 10-3/4" Caldera cartridge with a 2" male slip bottom (C-7451, OEM 033016) is not the same fitting as the 7" × 10-3/4" cartridge with a 2-11/16" open bottom (C-7350, OEM 1019401), even though both are listed at 50 sq ft with a handle on top.

What the catalogs list

A spa filter cartridge is a stack of pleated media between two end caps. The outside diameter and the overall length tell you whether the cartridge fits the well. The end caps tell you whether it seals. Unicel’s 2026 Replacement Guide and the Pleatco 2023 catalog both give those four fields for each cartridge: diameter, length, top and bottom^[5][2]. Square footage is a fifth field, and it describes media area, not the shape of the cap. Our square-foot guide covers that difference. This page is about the caps.

When you pull the old cartridge, look at each end before you measure across the pleats. The patterns that show up over and over in the spec tables are:

• Open both ends. A round hole in the top cap and a round hole in the bottom cap. The catalog prints the hole size, for example 1-15/16" or 2-1/8". The cartridge slides over a standpipe or sits between two sealing surfaces in the housing.
• Handle on top, thread on the bottom. The top is a molded cone, a bar, or a semi-circular handle you grab to pull the cartridge out. The bottom is a male thread, printed as MPT or as an SAE thread of a stated size.
• Handle on top, male slip on the bottom. The bottom is a smooth pipe-like stub, not a thread. Caldera’s older 7" cartridges are printed this way.
• Handle on top, open bottom. Common on later Caldera cartridges: you still lift it by a handle, but the bottom is an open hole with a stated inside diameter instead of a slip fitting.
• Closed or castle top. Some Sundance cartridges are printed with a castle top or a closed top and an open bottom of a specific size. The top is not a hole you can see through.

None of those descriptions is a guess about how a spa “usually” works. Each one is a column in the spec table for a particular part number^[5][2].

Same length, different bottom: Caldera’s 7" cartridges

Caldera’s Paradise, Utopia and Highland collections use several 7"-diameter cartridges with a semi-circular handle. Two lengths show up: 10-3/4" and 14-3/4". Inside each length, the bottom changed.

A tape measure across the pleats will not separate C-7451 from C-7350. You have to look at the bottom. The Unicel note on the C-7451 row says C-7350 can also be used, and the note on C-7479 says C-7375 can also be used^[5]. That is an interchange note from Unicel, not a statement that the bottoms are printed the same. If your old cartridge still has a 2" male slip stub, order the slip-bottom number and compare it with the cartridge in your hand. If the bottom is an open hole, order the open-bottom number. The longer pair is the same story at 14-3/4". Read Caldera filter bottoms before you buy for a Paradise, Utopia or Highland tub, and use the Caldera model-year lookup when you know the model.

Open both ends: the Hot Spring footprint

Many Hot Spring cartridges have no thread at all. C-6430 is 6" in diameter and 10-1/2" long, open 1-15/16" at both ends, rated 30 sq ft. Unicel, Pleatco and Filbur list it for OEM 31489 (also printed as 71825 and 71826), as Pleatco PWK30-4 and Filbur FC-3915^[5][2][31]. C-8465 is the larger open cartridge: 8-1/2" × 10-1/2", open 3" at both ends, 65 sq ft, OEM 31114 / 71827 / 71828, Pleatco PWK65, Filbur FC-3960^[5][2][31]. Both are open-end filters. They are not interchangeable with each other, because the diameter and the hole size differ. A full walk-through of which collection uses which one is in Hot Spring filter sizes by collection.

Open-end cartridges in the 4-15/16" family are just as specific. C-4335 is 4-15/16" × 9-1/4" with 2-1/8" open ends (OEM 74148, Pleatco PRB35-IN, 35 sq ft). C-4326 is 4-15/16" × 13-5/16" with the same 2-1/8" open ends, at 25 sq ft. The hole size matches. The length does not. Ordering by diameter alone would mix them.

Threads: MPT and SAE are both “threaded,” and they are not one size

Where the catalog says “male thread” or “MPT,” the bottom screws onto a fitting. Where it says “SAE thread,” it also screws on, but the thread series is the SAE thread printed for that part, not a generic pipe thread. The spec table gives the size either way^[5].

• Sundance 4CH-35, OEM 6540-486, is 4-5/8" × 16" with a molded cone handle and a 1-1/2" male MPT bottom, 40 sq ft.
• Bullfrog 8CH-951, OEM 10-2870, is an 8" cartridge 5-3/4" long with a semi-circular handle and a 2" SAE thread. Pleatco lists it as PBF36^[2].
• Jacuzzi 6CH-960 is 6-3/4" × 15-1/2" with a semi-circular handle and a 2" SAE thread. The look-alike 6CH-961 uses the same OEM numbers in Unicel’s table but is printed “open with slots” on top, and Unicel says it requires an injection-molded semi-circular handle that is not furnished^[5].

A 1-1/2" MPT bottom will not become a 2" SAE bottom because the lengths are close. If the old cartridge’s thread is worn, match the OEM number molded into the cap before you match the thread you can still see. Sundance and Jacuzzi look-alike filters goes through the pairs that share a body and not a fitting.

How to check the fitting on the cartridge in your hand

1. Turn the spa off before you open the filter well. Hot Spring and Jacuzzi both say to cut power first^[9][10].
2. Read any number on the end cap or the band. Search it from the homepage. A readable OEM, Unicel or Pleatco number beats a measurement.
3. If the number is gone, measure the outside diameter across the pleats and the overall length, including a handle or a threaded nipple. Then write down the top and the bottom in words: open (and the hole size, if you can measure the inside), handle, closed, slip, or thread. The measuring guide and the size lookup use the same fields the catalogs use.
4. Compare your notes with the top and bottom lines on the cartridge page, not only with the diameter and length.
5. If two pages share a size, open both. The one whose ends match is the one to order.

Pleatco’s cleaning sheet makes a related point about the media itself: a cartridge can look stained and still be the right part, and a white cartridge is not automatically a clean one^[3]. Stain is not a fitting. The end cap is.

What to do when the sources disagree about the hole

They sometimes do. On the Intex Type S1 cartridge, Unicel prints 2-1/16" open ends and Pleatco prints 1-5/8", while both print the body as 4-1/4" × 2-15/16"^[5][2]. Our Intex C-4317 page keeps both figures and tells you the body size is the part they agree on. When a page on this site says the catalogs disagree, order by the manufacturer’s part number and hold the new cartridge next to the old one before you drop it in the well.

Replacement intervals and cleaning are separate decisions. A correct fitting does not tell you how old the media is. Use how often to replace a hot tub filter and signs the filter is bad for that, and how to install a cartridge once the part number matches.

Questions

Sources

2. Pleatco 2023 Catalog (Pentair)
3. Pleatco FilterWash information sheet
4. Unicel: Maintenance & FAQ
5. Unicel 2026 Replacement Guide
9. Hot Spring Hot Spot Collection Owner’s Manual (2026)
10. Jacuzzi: Hot Tub Filter Cleaning Made Easy
31. Filbur Catalog & Reference Guide

This guide summarizes published manufacturer and public-health guidance; always follow your own spa’s owner’s manual. See our disclaimer.