I have had two cylinder heads develop a pin-hole coolant leak just under the thermostat housing. Anyone else have this happen? Causes?

 

I heard about that somewhere, I don't think it was here. Might have been an RV/Motorhome site.

I think the cause was the water rushing through the turn wore away the aluminum.

I'm also pretty sure it was fixed in later versions, but that might have only been with the higher-performance motor/heads.

What year is the truck/motor?

 

The engine is in a 2001 E450 passenger van. It occured on the first engine at about 150,000 mi - installed a reman long block- and it occured again after 40,000 mi.

 

Sounds like cavitation erosion. It's a common problem in diesels, not so common in gassers, but it does happen.

Basically coolant is rushing through the engine at a pretty good velocity usually and at high temps, near the threashhold of boil. If there are any air bubbles or pockets in the coolant for whatever reason -or- if there are any significant restrictions(blockages) in the passages, the velocity will increase, lowering the pressure. This will cause effervescence(slight boil) of the gases in the coolant. When this happens at high rates of flow, it will erode or eat away at the metal nearby.

Think of it like that constant drip off your roof that eats away at your driveway over a number of years.