You had a wrong impression.

Especially in a 351M or 400 (and I think 351C) .... you need the proper thermostat as already said because right under that thermostat is the bypass passage back to the water pump. As the engine heats, the right thermostat starts to open and ads it does, that "hat" mentioned also is pushed down to restrict that bypass so coolant flow is redirected through the thermostat and into the radiator top where it starts to cool as it flows down into the lower side of the radiator.

If no thermostat, then no restriction, and hot coolant that is carrying heat gathered from the block's water jackets around those 8 cylinders is pulled back into the water pump and recycled through the engine to .... get even hotter. Some little bit goes through the radiator, but not near enough to keep the engine temp down.

If you run a wrong thermostat, one without that "hat", you still have heated coolant being pulled back through the water pump by the unrestricted bypass passage and you have diminished cooling capacity.

You only want to restrict that passageway when the thermostat is open too, the whole thing is designed to keep coolant flowing with the water pump so it doesn't cavitate. If you didn't have the bypass, and the thermostat was closed, the water pump would spin it's impellers in coolant and just the coolant on the vanes of the impeller move and no cooling. So we have bypasses to keep coolant moving.

You also would benefit a bunch by having a shroud.

The point points up or towards the radiator.

Robert Shaw 333 series thermostat.

 

Great post and info ^^^^^^^

 

Alright guys its update time.

To my surprise the truck did have a thermostat in it, and it did have the "hat". However the one that I had picked up from the parts store was incorrect and wayyy to large. While the correct one is on order I put it together without a thermostat to see how she would do. The truck ran cold all day, even with spirited driving. Maybe the thermostat had gotten stuck?

I am still looking for a shroud and mulling over the electric fan swap idea. Wiring seems like a PITA though!

Thank you guys for the information!

 

It's possible the tstat got stuck, I've heard of it happening.

I checked on which shroud I have, and it's definitely the one from the diesel which won't be what you're looking for. I would recommend going with an electric fan. Wiring really isn't as bad as it seems (okay, it's a little hard when you're color blind lol). All you need is a nice relay, a switch, some wiring and some terminals and that's it. If you want, I can go back and look at exactly how I wired mine tomorrow and let you know. It's very straight forward though.

 

That would be really nice if it's only a tuck thermostat like it sounds like. Then you can take your time to decide on the fan option.
My truck cooled good with the mech fan and shroud, I'm only changing it because I'm putting in a 351w with a surpintine belt that has a reverse flow water which mean I need to change the fan to a reverse flow fan.

 

OP, what sort of outside temps are you seeing when you have this problem? Is your radiator original? I ask since I don't have a shroud and I'm debating whether to get one or go with an electric fan. So far I haven't had any temp problems, but I have a new 195 thermostat and a new radiator. Even in traffic this week with outside temps at 95, the my temp gauge sits between 195 and 200. But it will get hotter later this summer . . .

 

When the engine ran hot it was 80 or so during the day and mid 60's in the evening. I believe the radiator to be original.