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This is the portion that is the cap.
What inside the cap looks like.
Vac packs make nice stiff hardened plastic sheet.
Cut out larger than you need then pare down to fit, tighter than I did:
This is too loose, much tighter fit or the impeller will pull it out and make noise. If the plastic bows up from the fit, trim it carefully till it lays flat. Careful, this stuff is a springy leaper.
If the hole is too small it will also make noise and put heating strain on the motor.
With the plastic in place carefully penetrate it. I keep my little blade dullish except for the point. The starting point I made was on the "diameter" of the hole I want.
Gently slowly fold over and make a crease, then cut out the little hole.
Insert in cap, hold cap open-up and reattach to magneto housing with impeller. Cross fingers, plug in, listen for noise and watch flow.
Remember to start out with a smaller hole but not a TINY hole, the motor uses water flow for cooling. That square groove lets the water be sucked up along side the metal pin then spun out and wasted back into the flow, cooling the pin and making a hydraulic bearing of water pressure. This is the most common thing to bind up on almost all aquarium water pumps.
This is the magneto housing, pin in center. This is filled with water around the: