i know this is kinda lame but i just replaced the thermostat on my 95 f 150.
after i buttoned it up i started to check for leaks and i was pouring antifreeze on the floor of my garage.
i replaced the gasket as well but not sure as to why its leaking did i forget something?

 

This is a V8?

Sometimes the thermostat isn't installed in,, or slips out of the groove thats cast into the water outlet housing and won't allow for a good seal.. Also sometimes if a 'less quality' gasket is used(some cheap units have oval/elongated holes) it can squeeze out or seperate the gasket material during tightening creating a leak, more so if the thermostat isn't alighed into the casting seat.. Also, check the thermostat housing for flat across the surface from ear to ear during any replacements as well as for any cracks at the bolt ear to main body portion of the housing..

 

it seems like everything is good and flush. gone over with a razor blade for 20 min. the gasket i had (one of the stiff felt or cardstock kinda things got destroyed from the leak. i switched to a liquid gasket made for the thermo/water pump housing. ( does this stuff really work and how reliable is it )still have a leak but it has moved so i think i just missed a spot.