Need help fixing shower tap

[OM2]

Our shower tap doesn't stop the water, it keeps on turning.
We gently have to keep turning to the right until the water stops.

See below for a picture of the taps:


I took off the cover of the faulty tap - I can see a screw underneath, I guess this is where I need to start?

Someone told me the thread of cartridge inside has worn out.
They said I need one of these:

https://www.screwfix.com/p/flomasta-ceramic-tap-glands-24mm-x-2-pack/2961F

The problem is that there 3 different sizes on Screwfix: 9mm spline, 8mm spline and 10.5mm spline.

Is there anyway of knowing what size is needed without opening up?

Before opening tap: I've been told I need to shut off the water. The only way I can do this is outside on the pavement. (I saw the builders do this when we had our extension done a few years ago.)

Do I HAVE to stop the water?
I guess if I have to, then I have to.

EDIT: I was told I should replace both cartridges inside. The only thing is that the right hand side is for the water - this is the faulty one. The one on the left is for the temperature. Should I still replace both? The age of the unit is 10+ years old.

Thanks.

 

[oldbuffer]

1. Undo the screw inside and remove the knob. (No need to shut water off)
2. Measure the diameter of the splines on the shaft to which the knob fits. Needs to be an accurate measurement.
3. Count the number of splines. Use a felt tip pen to mark a start spline, then count round.
4. I'm afraid it highly unlikely that the cartridge will fit your shower, but, you never know!
5. If you know the make and model of the shower, you will probably be able get a cartridge to fit.
6. To remove the cartridge you will have to isolate the water.
7. I'd leave the temperature adjustment cartridge if it works. They are generally very expensive for what they are, and you would have to know the make and model.

 

[Madrab]

The tap on the right turns the water on and off the one on the left controls hot much hot and cold water is mixed.

If the one on the right isn't turning the water off properly then it's the only one that needs replaced. When it worked properly did you have to turn the right tap more than a 1/4 turn to turn the shower on and off? If so then the valve in your link isn't the correct one. You need a normal wind down valve

You will need to turn the water off to dismantle the tap and see what type/shape of valve it needs. With any luck it may just be a standard tap valve.
Maybe at the same time you want to look at getting an internal mains stop tap fitted.

 

[muggles]

Looks like a Trevi Therm Mk2. The Screwfix bits won't fit. You need...

https://www.showerspares.com/search/trevi-1-2-flow-cartridge-assembly-single-a952501nu11/

and

https://www.showerspares.com/search/trevi_therm_thermostatic_cartridge_assembly_s960134nu/

 

[OM2]

@muggles OMG, why would you scare with a link to a £140 cartridge!!
You might be right. When we moved in, we found odd brand names we'd never heard of like Villeroy and Boch.
Only later we found out these were really expensive brands.

@oldbuffer I think I might try what you say.

@Madrab It used to work perfectly. I think it was a 1/4 turn.

QUESTION: where would I find the brand name? Would it be on the underside of one of the plates for example? Just think I can prise open if needed.

 

[Madrab]

The T on the left control isn't just to signify temp, it stands for Trevi (Ideal Standard), could be a Therm or a Boost. Looking at it a bit further though, either or, it could be the valve(s) or the mechanism (there's a rod, gear linkage and cogs with clutches that turns on each valve).

I would advise you get an experienced plumber in to fault find it first as it's not straightforward.

 

[muggles]

@muggles OMG, why would you scare with a link to a £140 cartridge!!
You might be right. When we moved in, we found odd brand names we'd never heard of like Villeroy and Boch.
Only later we found out these were really expensive brands.

@oldbuffer I think I might try what you say.

@Madrab It used to work perfectly. I think it was a 1/4 turn.

QUESTION: where would I find the brand name? Would it be on the underside of one of the plates for example? Just think I can prise open if needed.

Well you did suggest that was what you wanted. A bit of Googling finds one for £90 though... https://www.ebay.co.uk/p/2803414928...i9-OsxXLjhLw2th2v4sc2Jpfv0ouIsLRoCy9UQAvD_BwE