I know minor and major diameters, but please explain what you are telling us above. Sure the 5/16 reamer aids in opening up the phenolic tell us what you are doing with this special tap .281 Maybe your wording was incorrect No one uses a tap the same diameter as the minor of the pin, nor will a .281 tap do anything in a .312 hole
Maybe you meant a .281 hole for your 3/8-10 tap that would work, then ream it with the 5/16
I'm not trying to argue with anyone, i'm trying to learn something here. The tip about the blo and sealer was good +