Hey Tim do not know if it fits a 55G bought it for a friend with a vinegar barrel. For wine in a 55 G I’d recommend a silicone bung definitely.

They are supposed to but did not fit snuggly for me. I tried everything. I would say they are a waste

Don't know I can't get my hands on mine now it's buried away. I bought it to put in a 5-gal barrel but I think it may have been too big for the 5-gal...memory fading. I ended up making a frankenbung out of a carboy silicone breather inside a larger universal with the bottom cut out. This bung says it's 1-27/32" aka 1.84" but doesn't say if that's the top or bottom. They need to update their description to include both measurements. A standard wine barrel bung hole is 2" so you want a bung that tapers from ~1.75 up to ~2.25" like these:

They do..