RT1139 vs RT1203 Thermostats
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3rd Gear
Joined: May 2022
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RT1139 vs RT1203 Thermostats
I’m getting ready to replace the thermostat on my 94 F-150 4.9 300 and there are two Motorcraft parts that seem nearly identical, but one is $7 (RT1139) and the other is $36 (RT1203). The former opens at 92° and the latter opens at 90°, but also sometimes at 92°. Does that really matter? Wondering if anyone has some insight
Tuned
Joined: Jul 2023
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I see something else that is different. Apparently the RT1203 has a rubber gasket on the circumference of the circular metal flange, the RT1139 does not. It appears O'Reilly's website does not have the RT1139, various Ford dealership sites I have tried says it's discontinued, the website of Summit Racing says it is unavailable.
Lead Driver
Joined: Jun 2021
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Heres the housing, the thermostat fits in the groove and the gasket is on the housing, not the thermostat
here is procedure
according to ford this is the thermostat, idk if its been superseded oranything (192)
Lead Driver
Joined: Jun 2021
Posts: 5,219
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assuming this is an accurate RT1203
this would seem to show the offset in the proper (this one) is also possibly in the RT1203, which just runs "2" degrees cooler. If the outer diameter is right is should work with the t stat housing as the service data shows I think the only reason for the o ring is for use on v8 engines with a housing that supports that, not sure what or when or which but yeah
Last edited by AuroraGirl; Nov 1, 2025 at 11:57 PM.
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