[toolboy62]

Most valve guides need to be reamed to fit. I am thinking that in the olden days, people would reuse old valves and sometimes they would want to polish the stems slightly undersize and ream the guides for a proper fit. Not sure why this is still done, but it is also possible that the guides themselves cannot be guaranteed to maintain the proper ID when they are pressed into the head, so making them undersize and reaming to fit after installation makes sense.

I am not a machinist, I just know enough to be dangerous.

nate