I had a 574 IH with a d239 engine and it would take hours to get it full of coolant. With wet sleeves, I didn’t like running it with an air pocket in there some place. The aspirin took care of that problem and ill explain below……. When changing a thermostat in an engine, press the flap open just enough to get an aspirin in to hold it open. Some engines will have an air pocket and can’t get the coolant system full until it’s run. With the aspirin, the coolant will fill the system before starting the engine. The aspirin will dissolve and the flap will close. Just a little trick I learned. Any feedback, let me know.