I have an 03 with the 5.4 the thermostat housing was leaking a little, over the last 3 months it leaked enough to fill the first 3 sparkplug tubes with antifreeze leading to a bad miss problem. So, I decided to replace all the plugs (95k miles) and I sucked and blew all the fluids out before removing the plugs.

On the thermostat, I see it goes into the round area, its the same size as the thermostat is round nice snug fit, my question is the gasket. Autozone gave me a red o ring type first, it was too small around to slip over the end on the thermostat housing. So I went back, then they give me a black gasket and the kid tells me it slips around the outside edge of the thermostat, the gasket has a small slit in it that allows it to fit around the outside edge of the thermostat. But, it won't fit into the hole this way, the diameter is too large.

How does this fit together and what gasket is the correct type gasket?

Thanks guys

 

i have a 2002, (which is the same setup as yours) and when i bought my thermostat it had a O-Ring on it already that sucked up around the rim of the thermostat when i tightened it

make sure you take the old O-Ring off before you put the new one in

 

I do use aftermarket parts but find that way to often anymore the aftermarket aren't the same or aren't designed the same and just don't work well at all, so if it doesn't work like the original, back it goes. If the manufacturer can't even make the part to fit correctly, what else is wrong with it?

 

all i have ever seen in these trucks is an o-ring that sits ON TOP of the thermosat and seals against the housing when tightend.

 

Exactly ^, that's what I did the last time, plus I've done this in the factory where your engine was most likely built.