This is a Holley Carburetor fuel inlet nut. It is brass and the Specifications are 9/16 x 24 thread. accepts a 3/8 fuel line with 5/8 x 18 fitting. This fits all Holley side feed carburetors, Holley 4010 and 4011 model carburetors as well. Demon Road and Road jrs require 1. All Speed, Mighty AND Race DEMONS require 2.