Just a WAG, but we usually tried for a .0015-.002" press fit on the guides we installed. Bronze guides get a little more of a press than cast iron. Once the new guide is installed, it gets reamed for the proper running fit for the valve. Heating the head and/or freezing the guide makes for a smoother installation, as it shrinks the guide and expands the hole in the head a little. Make sure to have some kind of positive stop in place to assure all the guides you install end up at a uniform depth, as it's easy to drive one too deep during the installation process. I used a flat washer on top of the guide and a stop sleeve around the new guide for for the washer to butt against.
Jerry

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