A coupler assembly includes a fluid fitting formed with a first bore at one end thereof and a second bore at an opposite end thereof, both of the bores being formed with female threads. A fluid adaptor includes a neck portion ending in a terminal portion larger in diameter than the neck portion. A fitting ring secures the fluid adaptor to the fitting. The fitting ring includes a split ring that fits over the terminal portion. The fitting ring is formed with a male thread, which mates with the female thread of the second bore of the fitting. The fitting ring has a first inner portion with a flat annular shape and a second inner portion with a curved annular shape.