I’ve got a new 40L Buffalo hot water boiler to use as a HLT in my new setup. I need to remove the crappy plastic tap and replace it with a decent ball valve.
Remove the tap and save the plastic washer to use with the new fittings. You’ll notice the tap hole isnt round, so you’ll need to round it out and slightly enlarge it for the 1/2 nipple to fit.
At this point I got carried away and forgot to take photos! Wrap PFTE tape around the nipple and fit it into the ball valve. Wrap more PFTE tape around the nipple where the back nut will attach. Put the original plastic washer onto the nipple and insert it into the boiler. Stick the rubber washer onto the nipple and then the back nut. Tighten! Lots!
Fill with water, heat it up and check for leaks. I was lucky, no leaks at all on the first attempt!