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Discuss Limited options for adding a boss to 90mm cast iron stack in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Hello,

I have the exact same issue as this chap had 4 years ago, however I am unable to post on that thread and no solution was ever arrived at (Cast Iron Boss for 40mm waste - https://www.plumbersforums.net/threads/cast-iron-boss-for-40mm-waste.89358/). I have already read a lot of threads on this forum, scoured the WWW for hours & spoken to plumbers merchants and even called McAlpine sales.

AIM: I have a combined rainwater and kitchen waste stack made of 90mm cast iron that I am adding an additional sink to.

The issue is that nobody appears to make a strap-on boss for a 90mm pipe. There are plenty of options for 110mm and 82mm. The only product I can find that might work is the McAlpine BOSS90TCAST-BL, however it isn't ideal as you are reduced to 22mm ID. McAlpine said it wouldn't work, but I think they have to say that, I can't see why the water wouldn't flow through, the sink would just drain a bit slowly.

The plumbers merchant said I might be able to use the McAlpine BOSSCONN82T-BL however he wasn't sure how good it would work on a 90mm pipe. McAlpine sales again flat out said it wouldn't work.

Does anyone know of any alternatives I might have missed? Any suggestions or advice would be appreciated.
Thank you
Rab
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Hi Shawn
Thanks for the reply. That is indeed the only part that I can find which is a possible solution. I referenced it in my original post:

The only product I can find that might work is the McAlpine BOSS90TCAST-BL, however it isn't ideal as you are reduced to 22mm ID. McAlpine said it wouldn't work, but I think they have to say that, I can't see why the water wouldn't flow through, the sink would just drain a bit slowly.

I am an engineer but not a plumber. Would the reduction in ID from a 40mm waste to 22mm be an issue?
Thanks.
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Basin ? Or kitchen sink


This would work but wouldn’t look the best as there would be a 5ish mm gap either side but the rubber part would seal
 

moonlight

Plumber
Gas Engineer
Advent Win
Is the stack internal or external? If its external might be worth considering an upgrade.
 
It is going to be a sink but outside, like a utility sink. So the stack is external too.
Post automatically merged:

I'll be the first to admit I don't know about pipe standards. Is 90mm even a pipe size? When I first measured I calculated the diameter based off the circumference I measured, which came to 87.5mm so I assumed it was a 3.5"pipe. I have just re-measured with a set of calipers and it is 86mm.

Have I potentially misidentified an 82mm pipe?
 

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Last edited:

Ben-gee

Esteemed
Plumber
I would use an 82mm strap boss.

Instead of solvent weld, use CT1.

If necessary use a longer bolt to join the arms together as you have a larger diameter pipe.

I have successfully used this method twice, in fact once I only had the type with the push fit arms with the ratchet teeth and they would not engage properly as pipe diameter too large - this was overcome by wiring the tabs together.

Both installations have been trouble and leak free for four years..(why did I say that...)

Dry fit everything first in case you need to source longer bolt etc.

Good luck.
 
FYI I completed the job and am posting here in case anyone else finds themselves in this position. In the end I used a McAllister BOSSCON82T, drilled a 57mm hole through the cast iron pipe with a Starett 57mm hole saw (rated for Cast Iron), and then ran 40mm polypipe to my basin waste. It was reassuring that the guy at the plumber's merchant agreed that the McAllister part would fit a 86mm pipe (which I think is just 82mm with a few mm each side of paint and primer to be honest). All tested with no leaks, so I'm pleased. A warning to anyone wanting to do similar, drilling the cast iron is a bugger and takes ages!
 

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