redring expression electric shower

[fisherman1]

I'm about to plumb the shower unit in and noticed that the elbow fitting is a hard plastic but the nut and olive are brass/copper.
The delivery copper pipe is 15mm.

Can I crank up the nut onto the plastic elbow and expect the copper olive to create a seal without fracturing the plastic.
How much force can I put on these plastic to copper joints to ensure they don't leak.

Should I also put a couple of turnes of PTFE tape around the olive and also in the plastic pipe tread.

 

[seco services]

the nut/olive will create a good seal no need for ptfe
i always prefit the nut/olive before install then fit the shower
and the nut/olive will already be tight on the plastic inlet just needs nipping up when all fitted.