Limited options for adding a boss to 90mm cast iron stack

[c.raab]

It is going to be a sink but outside, like a utility sink. So the stack is external too.
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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?

 

[Gasmk1]

Two Piece Cast Iron Soil Pipe Boss Connector - https://mcalpineplumbing.com/traps/condensate-traps-and-fittings/boss110cast-blc-two-piece-cast-iron-soil-pipe-boss-connector these are correct sizes never fitted one but you will need a good drill bit the right size

 

[Ben-gee]

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.

 

[c.raab]

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!