u/Feedback_Feeling

Netherlands. Bought a house last year and the previous owner left a Platinum Spas SR-OEM3 behind. First time changing the two filters and I’ve hit a wall.

What’s in the tub now
• Filters stamped Darlly 60401 (= SC714 / Unicel 6CH-940 / Pleatco PWW50 / Filbur FC-0359)
• ~150 mm diameter, ~210 mm long, semi-circular handle on top
• Bottom: a smooth female opening, 42–43 mm. No male thread protruding.

Every retailer and every cross-reference says the replacement for a 60401 is… a 60401. It has a male 1½” SAE coarse thread (~47 mm across the threads, 38 mm bore). Mine doesn’t have that. Mine is a plain hole that receives something from the tub side.

I went through the filter catalogues that publish bottom geometry in mm. Standard bottoms are: open holes at 28 / 49 / 51 / 52 / 54 / 55 mm, male SAE 1½” (38 bore / 47 OD), male SAE 1¼” (32 / 39), threadless slip SAE 1½” (37 / 48), female SAE 1½” (45–48 internal). Nothing sits at 42–43 mm.

The dealer who originally sold the tub told me the previous owner fitted blue adapters inside the black bayonet adapters in the footwell sockets, and that those blue parts are not original. Take the blue part out, he said, and the standard SC714 thread is exposed and normal filters fit. However the blue parts won’t move. They feel seized. He suggested pliers and turning left, but the black bayonet fitting is bonded to plumbing behind the shell and I’d rather not crack it and turn a €25 problem into a leak repair. He’s stopped replying to emails since May.

What I’ve tried / ruled out
• Hand force on the blue insert: no movement
• Searched EU and US catalogues for a 42–43 mm open-bottom filter: nothing exists
• Haven’t ordered anything yet as I didn’t want to buy the wrong filter twice

Is 42–43 mm simply what a 1½” female coarse thread measures across the thread crests? I’ve since read that the correct method is the widest inside diameter ignoring the threads (~47.6 mm). If so, I’ve been measuring the wrong thing this whole time and this is a much simpler problem than I thought.

Does anyone recognise these blue inserts as a specific aftermarket product?

Best way to free a seized blue adapter out of a black bayonet fitting without wrecking the fitting? My plan was: drain below the sockets, soak in white vinegar or citric acid for a few hours, then a strap wrench, or a wooden dowel cut to wedge inside the bore so the load is spread rather than concentrated on one edge.

Happy to measure anything specific if it helps narrow it down.

Thanks.