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there must be a sharp edge where the valve relief cut meets the chamber.
.060" us not sharp. There's the rub.
The proposed rule just puts numbers on what was the original intent of the rule. The .040" is actually a compromise beyond just saying sharp is 90º, with no margin for error. Pretty liberal.
I always stay out of these things...and I KNOW I should this time, too. But f-it, I'm bored.
The intent may have been different, however the actual words in the statement above are defining the intersection of two surfaces, and the shape that it is required to have (aka sharp):
1. The combustion chamber
2. The swept volume of the valve relief cut
The "chamber" is actually the open volume that is between the top of the piston and the cylinder head casting. The valve relief cut forms an edge with the casting *surface*, where the two volumes intersect (meet). The machined surface at the bottom of the relief cut is NOT "the chamber". The shape that the valve relief cut makes with the bottom surface isn't an edge (or that's a very loose and poor usage of the word), its an interior-corner. As such it can't be "...blended by hand, machined, or chemically processed to create a smooth transition". All of those processes will create an even "less-smooth" transition when applied to a concave shape. So, the rule doesn't even make sense.
This interpretation is further supported by the new wording. The same wording is present in the new rule, and a NEW sentence has been added to define the shape of the bottom of the cut.
So, its fine to say "there's the rub". And maybe that WAS the intent. I have no issue with codifying that intent. But, no finding was ever made that declared a cylinder head non-compliant to the rule. Let alone was there ever a rejected appeal of any such ruling where the imprecise wording was challenged. So, saying heads with a non-zero radius between the wall and the floor of the cut are ILLEGAL to the letter of the rule is just plain wrong.
The proposed rule does MORE than just put numbers to the original intent---it is defining a new limit on a previously UNSPECIFIED area. the previous rule said that the surface could be machined, but it did NOT define the shape of the cutter. Nor did the rule define the shape of the intersection between the wall and floor---it only defined the shape of the edge between the wall and the chamber (as sharp).
Quite frankly the whole idea that you can specify machining operations and allowed tolerances down to the fraction of a mm and degree USING PROSE and the occasional table is just obsurd. Nobody does that in the real world. We use these things call "control drawings" with defined tolerances, and explantory notes.