I concur with Brian662. I have a 15 gallon FIMCO that’s 12 years old that converted to an NPT thread bold drain cap after the OEM cap broke. I don’t recall the size, but it has been working for several years. Screw in the bolt about 75% of the way using a socket wrench. From then on, tighten it by hand only. While this is a back up sprayer, it gets used annually and so far it has never leaked.