The books all say there are two FE thermostat sizes. The later intakes have the

[Qikbbstang]

small one {+- 1-7/8"s} and the older intakes take the one large one {+- 2-1/4"}. Any ideas about what year did FoMoCo do this casting/size swap on the thermostat cover/radiator hose adapt castings?
            Is it cast in stone Large/Old and  Small/Later models?

         

[frankenfords]

From my experience, the change happened sometime in 1966 or 1967.  I had a 1966 dated 'S' iron intake at one time that used a large thermostat, and at another time a 1967 dated 'S' intake that took a small diameter thermostat. I also have an early Edelbrock F427 which uses a large thermostat and a provision for an oil fill tube, all the others I have seen use the small thermostat, and most don't have the oil fill tube knock out. I've never seen a 65 or earlier intake with a small thermostat, and never seen a 67 or later intake with a large thermostat.

[MT63AFX]

Could be a matter who (which foundry) cast them during the overlapping transition period, just a thought, as DIF were mainly doing 390 while CIF (CCC) was doing 428s or so it seems, Rod.