sean escribió: These are not "genuine Willys" parts, but are vintage period service parts, representative of the type used in CJ2A.
These are old school bellows type thermostats. The bellows is filled with some kind of liquid or gas (still unknown for sure) that expands w/temperature, opening the thermostat valve.
From left to right:
◦Harrison #1251 - 160º "High temperature", 1946 vintage, fits "Willys Jeep (military & civilian)".
◦Fulton #30-FHT - 155º "High temperature", fits "Willys and Jeep - ALL TRUCKS".
◦Harrison #312285 - 180º "Extra high temperature", 1950 vintage, fits "Willys 1937-1950 ALL".
Modern thermostats are a wax-pellet type. A wax pellet is contained w/in a copper slug, operating against a fixed rod, much like a hydraulic cylinder. The thermostat valve is fixed to the slug. When the wax expands w/temperature, the slug pulls the valve open. At cold temps, the valve is forced closed by the external spring.
There are technical differences between the two types that may be worth considering. YMMV.
1) Valve opening size. Both types use a circular valve, which when open creates a cylindrical passage.
◦The wax pellet valve is 1" diameter, and opens to .3", producing an area of .94 sq-in for coolant passage.
◦The bellows units are 1.375" diameter, and open to .4", producing an area of 1.73 sq-in for coolant passage.
Which one allows more coolant to flow through the radiator? Specially at the relatively low operating pressure (4 PSI) of the L-head, compared to the higher pressures (10-15 PSI) in cars that modern thermostats are designed for.
In lieu of the bellows type, it might be worth trying to find a modern type w/large valve, which has about the same passage area as a bellows:
2) Exposure to coolant. The L-head thermostat housing was designed for a bellows thermostat.
◦The bottom of a bellows unit is about even w/the top of the head. Coolant won't have to expand much to reach it.
◦A wax pellet sensor slug sits much higher within the housing. Coolant has to reach a higher level for the thermostat to begin working.
3) Failure mode. While both types are quite reliable, if they do fail ......
◦When a bellows type fails, it opens, allowing coolant to circulate.
◦When a wax pellet type fails, it REMAINS CLOSED!
Which would you rather have?
Sean