Either will do - copper ones are better if access means that its difficult to tighten the coupling firmly as they compress more easily.....i.e brass olives need slightly more force to compress them.
John
personally i change brass olives for copper ones. mainly because, i have removed a few compression fittings with brass olives on existing installs , and the olives have slid straight off! soft copper ones bite into the plastic better. as long as you dont over tighten them there isnt a problem.
a quick google found this.
Can I connect Speedfit Pipe directly in to a compression fitting?
Not all compression fittings are suitable for connection to plastic pipe. Compression fittings with a short pipe stop depth or hard olives should not be used with plastic pipe.
Use a ‘TSM’ Insert described in answer 1. This will enable the pipe to withstand the compressive pressure of the olive. Push the pipe with the insert up to the pipe stop within the fitting. The olive must be located along the length of pipe supported by the pipe insert. Hand tighten the compression nut then tighten the nut with a spanner approx one more turn. Softer copper olives are preferable to brass olives.
I have done a bit of seaching around my-self and it is quite hard to get a definitive answer .. Had a look at John Guest site , and they say copper is preferable to brass for plastic pipes ..
So to be on the safe side i think i will go with copper olives , just a bit of a pain -- new shower came with brass olives
The olive forms the seal. You'll see copper seals as washers on sump plugs and copper cylinder head gaskets, for example. I don't recall ever seeing a brass seal in any other application.
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