Cubist
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Such pipes are typically closed with a plug/cap. Check the diameter of the pipe, should be about 110mm. They cost about a tenner from the likes of Screwfix.
- Thread starter Garyivy
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Feltwell
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Yep - I'd use a drain test plug, if you put it in their search. You pop them in, wind the screw in the middle up and they expand and grip the pipe walls. If you then get problems with it being sealed and find that you need air admittance, the proper fix is extend your pipe and plumb in a durgo valve, aka air admittance valve, above ground. Or, if you're not so fussy and just want no big lumps of earth to get down there and aren't worried about some whiffs escaping, drill a few small holes in the test plug!
Feltwell
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Because when a big whoosh of stuff (technical term) is flushed down the loo, it needs some air behind it to travel, otherwise it'll try to create a vacuum behind it and pull air through the traps, i.e. the loo or sinks, and break the air seal - leading to nasty niffs from the sewer getting into the house.
Same principle as a plastic pop bottle full of water being turned upside down to empty it - it "glugs" as air has to be sucked in through the neck as it empties - but if you drilled a hole in the base of the bottle whilst it was upside down it would empty smoothly.