Grohe ceiling arm to shower rose leaks...

[crt]

Hi, I'm slowly completing a shower enclosure.

I've got a Grohe shower arm coming out of the ceiling which has screwed into it a 5" shower head (non-Grohe) using a 3/4" (head end) to 1/2" (arm end) adaptor. The problem is, the threaded end of the Grohe arm is only threaded part way round so that when the 3/4"-1/2" adaptor is screwed on, water spurts out of the gap.

I've tried wrapping PTFE liberally around the thread with only limited effect.

Would a thick smear of silicone seal the gap or do I need a washer or something else?

Many thanks.

 

[seco services]

whats on the part thats not threaded ?
not seen a grohe one thats not fully threaded.

have you a pic ?

 

[crt]

Sorry, haven't got a picture.
Grohe say they don't know why the flats are there. Problem is fixed for the time being however by liberal application of water hawk jointing compound on every available thread.