I was looking thru my stash of distributors for a spare and came up with one that I'm sure is a Y block distributor but the oil pump drive is smaller diamater. Why?

Smaller than what?  A Y-Block oil pump drive is 1/4" hex.

John - "The Hoosier Hurricane"

Good question. Looked at it again and tried the next smaller allen wrench which was too loose. Looked closer and discovered that the end of the distrbutor shaft was buggered. slightly at the very end. Will take a small file and clean it up.Should work then. Thanks. Sometimes in my old age what should be obvious is not.