Well... I feel that a threaded fitting is less than ideal. Sure, the threads will probably grip better than smooth rubber, but the diameter is actually smaller than the pipe. Also, it is possible that a tiny leak of water might follow the threads.
Here is what's supposed to be there:
This is a radiator hose piece for a race car, but the concept is the same. The raised ridge all the way around is what you need.
Come to think of it, this piece was hand-made by a radiator shop, and perhaps a local radiator shop can make what you need.
The ones you buy from NAPA or some such store will probably be iron and much thicker. That's what Blue Bird used for in-line splices in my bus.
Correct, I installed plastic fittings intended for cold water because it was all I had available, and we were getting ready to go on a long trip, and my passengers remembered the previous trip as too @#$%^&* cold. And it held for that 1000 mile trip! But the next time I drove the bus, I did not even get out of town before the "geyser" went off. (Moral of that story: When you do something for temporary use, remember to re-do it properly, soon!