Flush port fitting size

Does anyone know what the fitting/thread size is on the flush port for an older Honda BF75?. I could buy one or obviously use muffs but I am in the process of trying to get a 98 up and running and plan on doing a neutra salt Anne sump pump flushing procedure for several hours to see how much gunk I can get out. I had borescoped the cooling ports and the lower ports going into the head are pretty gross. Not plugged but in hopes a good flushing will knock some of it away. I was able to easily remove the buildup just by touching it.