How do I remove a stuck dome cover from old Caroma shower mixer?

[Cambridge]

I have replaced my shower head and rail and am trying to replace the mixer.  It is the exact same mixer tap as in the following video. 

https://www.youtube.com/watch?v=lOHHxNgU4Tc

I removed the handle no problem but cannot get the collar off.  I have tried boiling water, vinegar, soaking it in wd-40, strap wrench and force with water pump pliers.  It will not budge.  In the video it just screws off!

Any advice at all would be greatly appreciated.

[MitchellMc]

Welcome to the Bunnings Workshop community @Cambridge. It's wonderful to have you join us, and many thanks for your question about removing a wall mixer.

Given that you've tried several methods already, it sounds like you'll need to remove the dome destructively.

The first thing I'd do would be to clip a set of Vise-grips onto the front edge of the cover dome. That will then give you a surface that you can tap gently with a hammer in an anti-clock-wise fashion. This might be enough persuasion to budge it.

Failing the above working, I'd grab a hacksaw and put a slice in the dome until you can get a flathead screwdriver within the cut. Use the screwdriver to expand the cut enough that the dome will skip off the threads. Or, continue with the cuts on different sides until the dome falls apart. After looking at the video, it doesn't look like you'll damage anything under the dome. Just be careful that you don't hack apart the big copper nut, which you'll need to undo in the next step.

Have you tried using a pipe wrench that has teeth? The teeth might give you enough grip to turn the dome.

Let me mention one of our knowledgeable members, @TedBear, to see if he has any other nifty ideas.

Please keep us updated and let me know if you have questions.

Mitchell
 

[TedBear]

Hi @Cambridge , in addition to @MitchellMc 's good advice, I would add that you may need to leave that joint soaked in wd40 or similar for a few days,  respraying it each day. The fine build up of soap, etc can provide great sticking power. If possible, see if you can get some tooth floss into the join to help break up whatever is stuck in there.  Then try shaking it back & forth with the strap wrench or, if need be, with the snap lock pliers attached as described by Mitchell.  Sounds like a real nuisance.

[Cambridge]

Update:  

Got it off!  Took a good overnight soak in CLR and using dental floss - great suggestion!  - and a bit of tapping at with a screwdriver.

Discovered the new mixer I had purchased was going to be too small so now able to go source the right one.  Will post pic when complete.