Yup, "normal" spots for cracks. Refer to the link I posted. Junk head. I'm going to guess someone had a blown head gasket with an over-heat problem and didn't catch the cracks upon replacement.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Interesting, thanx. I had thought i caused the problem, but i just found an rpm calculator, and my highway rpms were btwn 3330 and 3700 tops, if i was going 90. That seems like the high end of ok for revving these engines? Also is there anything else i should do to prevent damage to new head other than rebuild injectors, pyro gauge, boost gauge just to check im not running too much, and maybe new timing chain?

One last question, when looking for a new head, is the hp difference in the head, or block btwn the earlier 119 hp vs 125 hp motors?

Did some checking and the n/a and turbo head casting is the same. But the valves, cam towers, cam, glow plugs, and prechambers are different. So the n/a head the machine shop has would need to have the supporting components changed over and grind the turbo valves and you should be good to go.
I know your concerned about overheating but as long as you have a working temperature gauge and check it should be enough.
Roddy

Sent from my SM-G900I using Tapatalk

I'd try to find a good used turbo head. All that conversion work on the NA head will be expensive. I have 240 heads loose but no 300 turbo heads on hand.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

While your head is off, have them smooth out the neck down area behind the valves. It's obvious once you have the valves out. You don't need to grind much to reduce temps and increase the flow efficiency.

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$60 OM617 Blank Exhaust Flanges
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No merc at the moment

Update... I ended up getting a turbo head rebuilt. Buttoned it all back up and drove from kansas to philadelphia, where it developed an intermittent overheating problem. New water pump and t stat and lots of burping had it running good for a month and then it overheated again. Parked it for a year, did a bunch of work on it, and drove it again for first time today. The nightmare continues! Well see if it randomly overheats soon...

My 1985 300D kept overheating, creeping up to 105 C at long stoplights. I tried many things - several clutch-fans, flushing w/ citric acid, radiator out sat w/ citric for a week then flushed, same w/ junkyard radiator, several T-stats, removed T-stat housing looking for blockage in the block (rag left in or such). Finally, I bought a new radiator when after-market ones were $130 (before cost $350). No more overheating, even on our 110 F days and climbing mountain grades. Popped the OE radiator open and found a thin yellowish coating blocking the top of some tubes. I had even run a stainless-steel grill brush across the top to free anything, and tried endoscope video but couldn't tell. It flaked off fairly easy, so not sure why the brush didn't work. Maybe had I ran it back and forth longer. Anyway, about time to replace the 35 yr old radiator anyway. You are likely using a Toyota radiator. Perhaps there are aftermarket aluminum ones available, or a new one off ebay that fits. BTW, I'm using Evans Waterless Coolant, but had switched back to pure water when flushing, and no difference.

One more thing is to insure there is no air trapped in the head. I don't know that it causes overheating, but seems it can make the temperature sensor read falsely high. At least that was my experience after a refill. Once I added a little more coolant to makeup for the drop after driving 3 miles, the temperature instantly dropped. Best to run the front up on ramps and crack the upper radiator hose just at the top until a little coolant dribbles out, but sounds like you tried that.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans