I've a need to join copper 1/2" and plastic 15mm hot and cold water
pipes. Will a straightforward 15mm pushfit coupler work?

I see Speedfit do a 1/2" to 15mm coupler, but I'd have thought to 0.1mm
or so between the 2 pipes would be within tolerance - and 1/2 pipe
varies anyway, depending on wall thickness.

I can partly answer my own question because I did try 15mm pushfit stops
on 1/2 pipe once and it didn't work (too loose, leaked) - although the
pipe may have been misshaped or something.
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Cheers, Rob