Hi @moloko,
Thank you for your question about hammer arrestors for your toilet.
As plumbing is a licenced and regulated industry, unless you are confident in your abilities, I'd suggest contacting a plumber to assist.
With this being said, the choice in water arrester will be relative to the size of the water supply line.
Something like this Sioux Chief 20mm AA Size Water Hammer Arrestor, which can be ordered through your local stores Special Orders desk should do the trick.
If you find that the connection to your water supply line is not the same diameter as this hammer arrestor, there is a range of brass reducers that can be used to adapt up or down to suit.
Ensure you use thread seal tape and multigrip pliers to get a good strong seal.
Let me know if you have any questions.
Jacob
