Regarding the three different SC heads, I've picked-up the same info from John Mummert's web page with what he's gleaned of cylinder head castings, Royce. The EDB-Ds and EDB-Es were production numbers while there is indication that the EDB-Cs were hand stamped and may have been (?) early (?) factory race application units. The 'early' designation is tied to lettering designation usually being assigned to letters ascending in order.
See JM's info at
http://www.ford-y-block.com/cylinderheadchart.htmIn reading James Eickman's 1984 Ford Y-Block book, the info on page 27 states that the first (production) EDB-D heads had no 'posting' "which sometimes failed in competition usage", then goes on to describe Vern Schumann's modification of these heads by screwing a bolt through the water jacket to support the backside of the combustion chamber wall, and then indicates that Ford subsequently "accomplished the same with a head casting EDB-E that incorporated an internal steel support rod".
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Regarding the suggestion on the FE Forum that 'E' code 285 HP ECZ-G heads may have been factory broached .060 to reduce the combustion chamber volume and raise the compression ratio... anything is possible, BUT the decks on the 'G' heads were thin at their normal production thickness whether produced during the '57 or (for the Mercury) the '58 model year.
The above said, my recollection is that the over-the-Ford parts-counter, late '56 power pack set-up 2x4 set-up included 'C' heads that had been broached .060 to increase their compression ratio...
Returning to the ECZ-G head information suggested on the FE Forum by a participant, in reconditioning work, both John Mummert and Ted Eaton have not recommended milling 'G' head surfaces more than .025 maximum for reliability reasons. I can't remember reading anything from Tim McMaster on this...
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York