How best to blank off waste pipe boss

[_doobrey]

I have got a 110 mm boss pipe in a waste stack which has an unused boss with nothing in it. This results in an open hole which leaks periodically.

An unrelated post, but it is along the lines of what is shown in the photo here: https://www.diynot.com/diy/threads/remove-blank-off-soil-pipe-boss.616095/ (Just imagine one of those bosses having a hole in it but no pipe connected. No access restrictions, though.)

I believe the diameter of these bosses is 67 mm(?) I expected that I could simply buy a blanking cap for it, and that these would be very common, but seems not. As an alternative I am thinking of getting a 40 mm boss adaptor plus a blank to fit the adaptor. This seems over-complicated, plus I am not sure of the size of the required blank. I suspect a 40 mm blank (e.g. from Screwfix) would would be sized to fit the inner diameter of a 40 mm pipe and therefore may be loose in a boss adaptor.

What is the best way to cap off one of these bosses?

 

[Collectors]

Just in case, i would fit a durgo valve. "Think that's how you spell it" Its quite a common item & it lets air into the pipe but not out, pluss they come in most sizes. If you dont have one of these it can restrict the flow of the drain if other pipes are using the same drain. They are around £20.
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[just pumps]

Just in case, i would fit a durgo valve. "Think that's how you spell it" Its quite a common item & it lets air into the pipe but not out, pluss they come in most sizes. If you dont have one of these it can restrict the flow of the drain if other pipes are using the same drain. They are around £20.
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Not required if stack vents outside.

 

[_doobrey]

It does vent outside, yes. The stack is outside.

Do I need an adaptor to fit the boss and a blank to fit the adaptor, or is there a more straightforward way of doing it?

 

[Hugh Jaleak]

Boss adaptor and appropriate size cap. The caps/access plugs are same outside diameter as the pipe, and are designed to go into a socket, so fill fit without issue.

I'd personally perhaps put a couple of cable ties around the pipe and boss adaptor, for belt and braces, to ensure it remains in position.

 

[_doobrey]

Many thanks. Noted.