The Reher article covers this nicely, but sort of brushes past what IMHO may be important but is very cylinder head-specific.
The clearance notch is described as a cylinder, or cylindrical section, with .050" radial clearance to the valve head circumference (OD +.100").
As a general rule this is pretty safe, but the safety is limited to engines with valve stem axes fairly close to vertical (Heron, BMC Mini, GMC L6, Buick L8 etc.).
Some of these can get away with less than .050" if the stiffness of the valve at overlap lift is high (low lift, tight stem-to-guide, long guide, short stem length to the guide, generous stem-to-head radius) since this minimizes "ringing" which is one reason for more clearance than a simple extension of the head shape into the dome.
The other use for the .050" is not actually discussed (unless I went right past it?): it's slack to allow for a non-vertical valve to shuffle slightly while opening/closing.
This is the part that I thought needed more attention.
An engine with high included valve angle (Jaguar L6, Chrysler hemis, Harley Sportster, panhead, shovelhead) has considerable sideways motion even at small lifts, and if clay is used the valve head will not depress the clay (as a slow impact) but instead plow it out of the way in 2 planes.
Luckily, the math is easy to do and very accurate if you know your valve angle. Using an obvious easy choice: 45°, the sideways motion is the cosine of that angle × the calculated or measured depth for lift only.
E.g., a 45° valve lifts .200" intrusion below the upper dome surface during overlap. The notch depth is .200" (duh!) measured on the stem axis (not the bore axis), plus whatever clearance to the bottom of the notch you need.
The notch width is the valve head OD + whatever ringing space you need (.050" radial, etc.).
The length of the notch is the width (as above) + (.200 × the cosine of 45°, or .707) = .141". Note that this is more than the radial clearance, and a very small error will lead to breakage. The valves generally open toward the bore CL but there are exceptions.
To sum up: heads with shallow stem axes may need less than the .050" radial clearance suggested (and have small chamber volume), since the valve head will follow the notch location closely.
Heads with high angles need either a relief in a more complex geometric shape (is there a term for 2 identical cylinders partially overlapping?), or a cylindrical notch defined as valve head OD + radial clearance + lift × cosine because of the scrub effect across the piston.
Those familiar with Excel can easily write a sheet that allows slider or check box selection of stem axes (they're not always identical - 426 is one), valve head OD, lift at overlap, and radial clearance, and produces the notch length and width. If the notch is a regular shape, it could even calculate the volume...
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