Let's discuss valve geometry try and explain this mystery of engineering or lack thereof.
You engine builders already know all this stuff but for the novices I'll try and explain how all these angles get dangled.
”Valve Geometry” is simply the position that the roller of the rocker arm rides on the valve tip, the roller should ride in the center of the valve tip through the cycle. But, because the rocker arm rotates on an axis and the valve cycles on a plain or straight line they move across the valve tip and are not stationary or pinpoint but rather they sweep across the valve tip. Good geometry will keep the roller centered through the cycle. Bad geometry will allow the roller to ride either on the front edge or back edge of the valve tip and cause severe side load on the valve and guide with eventual failure to one or both
You have 2 things that remain constant on a shaft rocker system...the height of the valve tip and the shaft centerline. Now, if you think about a stud rocker system as you adjust the valve lash you are actually changing the elevation of the rocker arm pivot point with respect to the valve tip or. …changing the valve geometry. Can you visualize the difference here?
The only way to change the geometry on a shaft system if the valve tip has been raised (long valves) is to raise the shaft, which changes the relationship or angle that the valve tip intersects the center of the pivot point or rocker shaft.
So with this explained you can now understand that cutting, surfacing or angle milling the head has nothing to do with the geometry between the rocker arm and valve tip, the problem of push rod length now becomes the issue. As you move the head closer to the cam the pushrod length needs to corrected to make up for the movement. If longer valves or aftermarket rocker arms are used it's critical to mock up the head and check for correct contact point on the valve tip, this may require in extreme cases that the shaft itself is shimmed up to correct the geometry. If this becomes necessary we use mock-up shims until we get it right then cut the correct thickness and shape of shim material and drill a hole in the center so the hold down bolt will lock it in place.
Once the head is torked to the finished short block the task of measuring the correct pushrod length begins. Using Prussian Blue (Or a simple Sharpie pen) available at most good parts houses, coat the top of the valve tip and adjust the push rod to zero lash using a solid lifter. Now rotate the engine until the valve goes through a complete cycle. Re-coat the valve tip and adjust the pushrod and lash until the cycle of the rocker arm over the valve leaves a wear pattern that is centered on the valve tip.
Measure the pushrod and order yourself a set of Smith Brothers Push rods and you’re done.