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So I decided to change the thermostat on my 1981 F250 with a 351M. When I took off the housing this was the thermostat that was in it:
From my research that looks more like a Windsor thermostat. The ones that I ordered look like this:
they both for into the little recess for the flange. I am just wondering if I messed up with my parts or if I have been driving with the wrong thermostat.
The 335-series 351M/400 block and the 351C use a unique thermostat, it looks to me like you've been using the wrong one. Absolutely Chris, the OP's truck had the wrong thermostat installed, it's probably a 302/351W stat.
D0AZ-8575-A .. 192 degree Thermostat (Motorcraft RT-139) / Obsolete ~ Available NOS and from auto parts stores (Robert Shaw brand).
1970/74 351C / 1971/82 400 / 1975/82 351M
351C only installed in 1970/74 cars / 351M introduced in 1975 cars, in 1977 F100/350, in 1978 Bronco / 351M/400 introduced in 1980 E250/350.
351M/400 not available in cars after 1979 / 351M/400 cancelled at the end of the 1982 model run.
This thermostat may be hard to find. Some peeps don't know there's a difference between 351C/351M & 351W, go to auto parts stores, ask for a 351 stat, end up with a 351W stat.
D0AZ-8575-A .. 192 degree Thermostat (Motorcraft RT-139) / Obsolete ~ Available NOS and from auto parts stores (Robert Shaw brand).
1970/74 351C / 1971/82 400 / 1975/82 351M
351C only installed in 1970/74 cars / 351M introduced in 1975 cars, in 1977 F100/350, in 1978 Bronco / 351M/400 introduced in 1980 E250/350.
351M/400 not available in cars after 1979 / 351M/400 cancelled at the end of the 1982 model run.
This thermostat may be hard to find. Some peeps don't know there's a difference between 351C/351M & 351W, go to auto parts stores, ask for a 351 stat, end up with a 351W stat.
Motorcraft is listing a new stat for the 335 series the 190° RT1167 also used in the Explorer Sport Trac with the 4.0L
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