Is there good and bad PTFE tape?

[Hot&Cold]

I bet that's where I went wrong!! I was putting an equal amount on all the threads

It can seal your way but your shower mixer could have high mains water pressure to cope with.
Have found on your type of mixer internal thread to be fairly loose,a lot of play when screwing fittings into and eventually water will find its way out of the threads but the ptfe flange will remain water tight.

Others will have different techniques but on your type of shower,the straight coupler as shown works for me.

Rad valve threads will be no problem youl be an expert by then.

 

[Hot&Cold]

buy some locktite 55

Never used it but heard good reviews,just progressed from hemp & jointing compound to ptfe,

Ian,would you consider fitting a better shower valve ?
Bristan Artisan is a thermostatic mixer&diverter recessed unit and have fitted a few,see installation pdf here.https://images.victorianplumbing.co.uk/images/pdf/AR3-SHCDIV-C.PDF

 

[Alec_t]

maybe one part is parallel and the other is tapered.

I wondered that. In the post #9 pics the fitting looks to have parallel threads.

 

[Ian H]

I would consider the Bristan if this one ever goes wrong. I could hang a picture of my mother in law on the opposite side to remind me what a ******* job it was!!