I have a 1990 ford ranger 4.0 ohv v6
Long story short I had cracked head problems, internal coolant mix problems and still having problems but to move forward I need to understand the engine more before I spend more money on the this engine. So the heads have a design flaw right the metal wasnt dense enough and the water jackets were to small. so my question is does the block have the same problems, and what about the lower intake?

 

No, the block was fine on the 4.0ls

The head issue was not a problem unless engine was over heated, it didn't cause overheating
The cooling passages are large enough.
The "flaw" is that the metal between the valve seats was thin, which is not an issue at regular operating temps

When any engine is overheated the metal in the heads get the hottest, because they are at the top of engine and because loss of coolant means higher points lose coolant first
And metal expands when heated, like anything else
This is why head gasket "blow" when engines are overheated, the head metal expands and since its held down by head bolts it can only expand DOWN and it crushes the head gasket between head and block

With the 4.0l heads the expanding metal can cause the thin area to crack, so its not a "chicken or the egg" thing, the engine must always be overheated FIRST and only then would a head crack, and its not an instant thing, it has to be way overheated for 10+ minutes, so pull over do NOT try to "make it home"
Let engine cool off then limp home if possible in short hops

And just FYI, all heads will crack if overheated long enough, always have, always will, lol
Its just that the 4.0l OHV is less forgiving than other heads