The manual says 7/8", but it's 1"-8. If it's like mine was, there's too much crud packed in it for the 1" tap to go in. I had to poke at them with a long screwdriver to loosen everything up and then blow them out with a long air nozzle - wear safety glasses because lots of stuff will be flying out. Put lots of WD-40 or similar on the tap.
Jon