Blok-Chek and Compression check
Blok-Chek
Blok-Chek was developed by P&G Manufacturing in the 60's. Since it was developed,other companies have marketed similar products. Most radiator shops have a testing device that is similar. This is the first test I recommend if you are experiencing overheating, diminishing power, or other engine problem symptoms in a Ford flathead.
These Blok-Cheks are typically a clear cylinder that has a one way valve and an adapter to the radiator neck on the bottom. The test fluid is held in the middle. Unused testing fluid is blue, but it turns yellow in the presence of combustion gases. On the top, there is an attachment for the included rubber bulb to suck air through the adapter, then the one way valve, and finally through the test fluid. If the air pulled through the blue test fluid has hydrocarbons in it, the test fluid turns yellow. Yellow is bad. The fluid may be used with the engine running different throttle openings and loads to find the circumstances that create the problem.
There are other ways to check for combustion gases in the coolant. One is to warm the engine up, take off the fan belt, restart the engine, and watch the coolant in the top tank for air bubbles upon sudden throttle opening. Of course you shouldn't run your engine very long without the water pumps turning.
You may also use a smog checking sniffer to check for hydrocarbons above the coolant in the top tank. They are very sensitive, so they may give false positives. If you choose this method of testing for hydrocarbons, use an experienced technician who knows what the "background" or baseline hydrocarbon levels are. This will change depending on what coolant mixture you are using.
So what does this all mean? Combustion gases in the coolant mean a leak out of the engine's combustion chambers. A leaky head gasket, a cracked head, or a cracked block are the probable areas of concern. Typically these leaks will be aggravated by wide throttle openings and lower RPM.
If your engine idles without overheating, but the faster you go, the hotter it gets, this may be the problem. One of these tests may find it for you.
Compression Check
Compression testing is usually done to determine the health on an engine. Also, it may point to some particular problem or multiple problems. Generally, a compression test is done on an engine that is at operating temperature and with the throttle valve(s) open. For safety, disconnect the coil secondary lead until you have closed the throttle valve(s).
The tester is a pressure gauge that either threads into or presses into a spark plug hole. Most compression testers hold the highest reading achieved. After cranking through three compression peaks, you will usually have reached a practical point to take the reading. The results will be more comparable if the test is performed the same way on each cylinder. Write down the results as you go -- you need to do 16 readings on your V8's.
After doing the first round "as is", add about one teaspoon of oil to each cylinder through the spark plug hole. Try to keep the quantity is each hole consistent throughout. If you add too much, the readings will go up due to the oil volume changing the mechanical compression ratio. Crank the engine a few times to spread the oil throughout each cylinder.
You should be able to do all 16 readings while the engine is still warm. The "without added oil" readings should be in the ballpark of your engine's compression ratio times atmospheric pressure, about 15 psi. All readings should be within a 10% range. The "with oil added" readings typically will be a little higher.
Any very low or zero readings may indicate a burnt valve or a badly damaged piston. Rotate the engine to close both valves in the cylinder and you can hook up compressed air to those cylinders and listen at the carburetor, tailpipe, or oil breather to see where the leak may be.
Readings from worn piston rings will usually come up to about normal with the addition of oil. Leaky valves will not usually be affected by the addition of oil. Another problem not affected by the addition of oil is valve lash so tight that the valve can't close.
Don't forget to close the throttle and reattach the springs and/or throttle rod. Use anti-seize on the sparkplug threads when reinstalling, especially on aluminum heads.
If you can't start the engine at all, a compression test may still be of value. On a cold engine, if you can't get 60 psi on each cylinder, you may have internal engine problems that will keep it from starting easily. Cylinders with no compression may have valves stuck open. Sometimes a light oil or penetrating oil may free the valve. You can sometimes close a valve that is stuck open by working through the sparkplug hole.
If there is visible coolant in the oil, or the oil drained from the pan is milky or what we sometimes call "mayonnaise", the compression tests I describe here may not be needed to find your underlying overheating or low performance problem. These are also signs of a leaking head gasket, a cracked head or a cracked block, requiring a teardown and possibly rebuild on the engine. 37-48 water pumps can fail, leaking coolant into the crankcase causing water in the oil also.
Good luck on your diagnostic work; you may make "Detective of Old Engines" if you can perform these tests and assess the results.