Howdy All:
Induction seat hardening begin to appear on D4 castings on the intake valves only. D5 castings frequently got induction hardening on both intake and exhaust seats. Hardened seats, both intake and exhaust, are a good bet in D7 castings with the increase in intake size. As always with Ford, never say always. The change is impossible to date. A D7 casting, with 1.75" intake valves is a good indication that this head will have hardened valve seats. If an earlier head has hardened inserts it has likely been rebuilt and had inserts added.
It is difficult to determine if induction hardening has occurred in a stock IEM head since it is a heat treatment of the cast head metal. If the area of the seat is very clean ocassionally some discoloration is visable. Induction hardening is not a very deep process. When in doubt have hardened seat inserts installed and end the worry.
The tricky part of the casting code is that a C5xx was used on both 170s & 200, but only the castings used on the 170 have the smaller, kidney bean chamber. The only way to, for sure, identify a "Kidney bean" chambered head is visual ID. Measuring volume/cubic centimeters can help if the head has not already been machined. The mysteries of casting codes continues to unfold. Casting codes are a good generality, but exceptions constantly appear.
The C9xx ('69) casting with the "M" suffix is a fairly sure indication that this was a flat-topped log head with a 1.75" carb hole. These heads can be found on both 200s and 250s.
What brand head gaskets have you been using? When removed what area of the head gasket shows the damage? Do any of the head bolts show signs of bottoming when removed?
Adios, David