Valve seat angles on valve vs. seat

A general question I have asked myself for quite some time is, why do the valve seat angles per engine manufactures specification need to be slightly different on the seat in the head compared the seat on valve itself?

For example the valve seat in the head needs to be exactly 45°, but the corresponding angle on the valve should be cut at 45°30'.

Why that half a degree difference?

Is it common to do it exactly like that, or do machine shops just cut them both to 45° on the seating surfaces?

I fully understand the practice about the inner and outer correction angles to adjust the width of the seating surface for getting the area for the sealing and heat transfer right. But just that half a degree difference on the actual contact surface makes my head scratch.

Ok I think I found the answer myself. I looks like it is to prevent reverse interference. Because of manufacturing tolerances on the angles the possible interference must be to the "good" site. I found this info detailed described here in the chapter "valve to valve seat interference (45° seat vs. 46° seat)":

http://www.newaymfg.com/cutters

Interesting information Silas. Thanks for sharing!

Yup, that's as I understood it.