I went to install my mechanical temp guage and pulled the 3/4" plug off the runner that goes between the front of the cyl heads on my Edlebrock intake. No sign of water!! I could of sworn that was a coolant passage on the last 400 I worked on.
Maybe that's why it's always started to get hot with the a/c under load? I'll bet the PO installed the gaskets backwards. From what I remember the water is supposed to circulate across the front of the manifold. Let me know what you think, I don't want to pull the stinking manifold off unless I have to. Thanks.

 

ALL 400 manifolds are dry, no water in them anywhere.

You will need a thermostat housing with the proper port.

 

The factory sending unit is at the front right side of the engine, by the alternator.

 

Crap. I swear on the last 400 I had that runner was a coolant passage. I realize where the sending unit is, I wanted to install the mechanical guage to compare it to the Ford setup. How many differant T stat housings did they offer? Mine has 2 plugs in it but they appear to be machine thread, not pipe. The sending unit for my aux guage won't fit.

Does the stock manifold run coolant through that passage?

 

I put my aftermarket gauge in the stock location next to the water pump and moved the stock sender to the gooseneck. This setup will tell give you some indication of the thermo opening. It will also read a bit high on the stock gauge at times.

 

For some reason the guage I got from Summit won't fit in the sending unit hole in the block/head area where the stock one goes. It's too small and the supplied adapter is way too big. I'd just mount it in the T stat housing if I could get the right adapters. Are the holes in the T stat housing NPT?

 

No stock or aftermarket 351C/M/400 intake has cooling passages, never have...

The threaded holes are NPT.

 

Then what's the runner for that goes between the heads? It's on the very front of the intake? Not an intake passage from what I can see, either for coolant or EGR system, but there is no EGR port on this intake.

 

It is for air injection, you can read about it at:
http://home.earthlink.net/~bubbaf250/
Or any number of threads here.

 

I tapped the heater outlet pipe hole after removing the pipe. I think that it was 3/8" NPT. Then I installed a short nipple and a 'tee' fitting. I put the second temp sensor in the top of the 'tee', and used a hose barb in the center to connect my heater hose.

It works OK, but it is better when the heater valve is open and water circulates.

I needed to use the factory water temp location for the EFI Engine Cooling Temperature sensor.

 

I'll probbably just tap a heater hose as as stated. It'll get a decent reading to see how accurate the stock guage is. I want to make sure it's accurate before I try and use it for my upcoming Cummins swap.

 

Just heat water very very slowly in a pan with the sender and a good thermometer elevated above the bottom.