flash7210 wrote:
The head cracks the way it does because of heat.
Hot exhaust valve closes, transferring its heat to the hardened valve seat, then to the aluminum, and finally to the water jacket.
If there is not enough coolant flow around the exhaust valve areas, or if steam pockets form in the water jacket, these areas will become heat stressed and crack.
And it is possible to have small localized steam pockets without any indication of overheating or rise in temperature.
In my case, I am convinced that the cause was because of poor coolant flow due to my choice of using a in-line thermostat in conjunction with a failed factory thermostat. Where the factory thermostat was opening too early, blocking the bypass outlet, and the in-line remained closed and blocked all coolant flow for a short period of time.
But if the OP was using the HDS thermostat then that might throw my theory out the window. (Although I still stand by it)
Still, casting flaws and poor design are possibilities too.
But the root cause is heat. (Same for broken exhaust valves)
So, create a free flowing exhaust to allow all that heat to escape quickly.
And let your engine run as cool as comfortably possible. A 195 thermostat might be too hot.
I’ve used both a 160 and a 180 thermostat with no ill effects.
Aluminum head/iron block engine architecture is relegated to being used in consumer grade passenger vehicle and perhaps boating applications. The commercial trucking industry, the military, and people involved in commercial water transportation would not stand for the headaches caused by two differing metals expanding at two different rates... it is iron head/iron block architecture only with the camshafts being driven by gears, (as it should be for everyone!).
What we have with the V.M. Motori line of automobile engines are what I call "delicate" engineering. The issues that come into play regarding the differences in thermal expansion of the cylinder head vs. the block are only partially solved by things like torque-to-yield bolts. It is much more problematic to make modifications to engines like these than traditional diesel engines without running into consequences. As such, when modifications like the ill-advised inline thermostat are employed, a lot of bad outcomes can be expected.
The HDS Model 001 will not cure the problem of delicate engineering in the R425 and R428 engines, but it will allow you to raise engine operating temperatures safely because it makes no changes to how the cooling system functions. In other words, it will not make the situation any worse; this is something that can not be said of the inline thermostat solution.
What these engines really need are cylinder head upgrade replacements made of IRON.