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Yes it is, sadly. I did some good research for a good thermostat and the Motorcraft RT1139 is the best one I could find. Cost me $13 though but it's a USA part and works perfectly.
I bought mine on eBay. Cheapest place I could find it. It came from a Ford dealer in Illinois I believe. I noticed the body is quite a bit thicker over the parts store thermostat I had.
Should fit just fine. This is actually the seller I bought mine from.
http://www.ebay.com/itm/MOTORCRAFT-RT1139-190f-88c-Thermostat-/220814932940?pt=Motors_Car_Truck_Parts_Accessories&hash=item3369987bcc&vxp=mtr
Says it fits 1983 300 Six trucks, no reason why it shouldn't fit an 85. They all use the same thermostat to my knowledge.
Yeah, thats the one I saw. I guess I'll just run the 160 until I can get one or just go get a regular one. Well, gotta get up early before the sun hits (110 tomorrow) to replace the jacked-up water pump I was sold.
The 4.9 hoses didnt come with one (Oreilly or AZ). The NAPA ones on there, didn't have them either and were pretty worn. I went w/the better priced AZ ones.
YOU NEED THAT SPRING IN THE LOWER RADIATOR HOSE!. Some new hoses don't come with them, so you have to pull the old one out of the old hose and stick it in the new hose. If you get a overheating problem going down the highway, it's because you didn't put the spring in the lower hose. At high rpms and temps, the hose gets very flexible and will suck shut and restrict waterflow.