How to repair valve guide boss

The factory “guide” is just a hole drilled in the iron.
What you have are bronze repair sleeves installed.
The original guide bore is drilled and reamed, and the repair sleeve is pressed in.
If the reamer is cutting to a slightly small finished size(getting worn out), there is a little more press on the new repair sleeve....... and you end up with a cracked boss.
Sometimes they just crack even when everything is “right”.

The guide top needs to be short enough to provide adequate clearance between the retainer and the seal for whatever lift cam you’re going to use.
I like to have about .100 clearance, but .060 is adequate.
I prefer to have a little extra in case there’s a cam swap to something with a little more lift at some point in the future.