I've just had to replace the innards of a close-coupled toilet cistern, which necessitated removing and 'uncoupling' it from the toilet. The actual replacement of the components went fine, however I was worried about re-using the original foam doughnut which sealed the cistern to the toilet, as it turned out to be quite deformed. And sure enough, a minor flood ensued when I flushed the toilet.
I've just been out and bought a new doughnut from the local plumber's merchant - the only one they had in stock - it was a 1.5" i.d. foam round ring (unlike the original which had hexagonal indents to match the plastic nut); the new ring was nothing like as snug. I fitted that (I could feel the foam compressing as I tightened up the two fixing bolts under the cistern) but just the same result - flooded bathroom.
What's the trick here? Do I need a bespoke doughnut which fits my cistern or something? Or should I just cover everything with sealant? plumber's mait?