More information has arrived. Last night I reviewed, Thermostat Issues and Selection
, and found that Don Elliott had included information regarding the melt-out temperature of the lead plugs and other temperature characteristics of the OEM UE36600 thermostat. Given that it's been a couple of years since we worked on this research, and that part was "his part," I e-mailed asking for specifics regarding where the data came from:
Sure, the 124 C temp was cited in TSD 4700 page L5-1 middle of the left hand column. The Crewe UE thermostat begins to open at 88 C and according to this same page is fully open at 102 C. The normal operating temperature is given as 99 to 102 C.
[I should also note that there is an error in the Shadow series documentation that repeatedly says the thermostat opens at 80°C, which has never been correct and apparently was never caught over multiple decades of reprinting.
Since all of the temperature regulation is controlled by the thermostat, at least if all is functioning as designed, the same data should apply to the SY cars as well since they all use the same thermostat.
This means that the normal "settled down" operating range is approximately 210°-216°F. There is, of course, the potential for a bit of "slop" at the top of a few degrees if the individual thermostat in question doesn't reach full closure at the noted temperature. They aren't precision instruments and while they are individually consistent in their behavior, different examples can vary +/-5% and still be considered OK. I doubt that there is any testing on these things at the point of production, so if you want to know what your current or intended replacement thermostat's temperature response actually is you'll have to test it out yourself on the stove top.
Given that the plug melt-out point is very well above the normal operating temperature range, I fully expect that there can be some "spikes" that are above that range under demanding conditions in hot weather but that should never even come close to getting near the melt-out point, which is a full 40 degrees F above the top of the ideal normal operating range.
FYI, there's no good reason to replace the fan clutch in these cars with used ones. New ones that are commonly available are activated at the correct temperatures. These are documented in the RR & Bentley Parts, Repair, Restoration & Other Resources Compilation
as well as in threads on this forum and the RROC-Australia Discussion Forums