When fan clutches first came out, they were the centrifugal type, which cut-out above a certain engine rpm level. Identified by no thermostatic spring to regulate internal fluid pressure.
When the thermostatic versions came out in the 1963 era, they were considered "upscale" as they could better modulate fan function to the demands of the vehicle cooling system vs. engine rpm. By that time, the earlier centrifugal versions were relegated to the "bargain basement" ares for people who couldn't afford the little bit extra price of the thermostatic units at replacement time.
There ARE some ways to tweak the OEM clutch's performance, seems to me. If you want a little bit quicker heat response, then carefully disengage the end of the thermostatic coil and put it on the tighter side of the retained area, for a bit more tightness. When I did this, it did not seem to make much difference on our Chrysler, but I knew it should, if it needed it.
On our '66 Newport 383 factory a/c car, if the coolant was at the spec level and the radiator (26" in this case) was not gunked-up with deposits, there was NO reason for the "HOT" light to ever illuminate. Those cooling systems just worked well by design. No trailer hitches or towing involved, just driving down the streets/highways normally at up to 90mph (where safe to do so).
Our service station guy (whose favorite cars were '65 Fury III 4-dr sedans, as he had a son and daughter) told me onetime that if a pressure-related leak might occur, to carefully release the pressure and then let the radiator cap be at the last safety catch to is would stay installed, but not allow any pressure to build up, and no coolant would be lost. From either the pressure-related leak or from the filler neck. This was on the old round-top radiator tanks. When I tried it on a square-top 1980s version, didn't work so well. I figured he knew what he was talking about as he'd driven/been around these cars much more than I had been, at that time. Proceed at your own risk. FWIW
Enjoy!
CBODY67
