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I'm quite certain on the mm for the aerator being 20 mm as I used a 2 dollar coin and backwards searched that etc and then narrowed it down to 20. But I've noticed that there's no threading? The images I provided below hopefully help out with my question for what aerator I'm meant to get.
Hi @JR4,
Welcome to the Bunnings Workshop community, it is wonderful to have you with us.
The calcium or limescale buildup that you can see around the bottom cylindrical piece implies there is a seam here that small amounts of moisture have come through.

I suspect that this bottom piece can be removed to reveal the thread you are looking for.
Grab a pair of multigrip pliers and a rag to wrap around the tap, then grasp the bottom cylindrical piece and rotate it anticlockwise. I imagine it will come out.
If you think it will come off, but it is stuck in place, soaking it with some CLR should help to loosen things up.
Give this a try and let me know how you go.
Jacob
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