900. Head gasket? Cracked head? Ideas???

[Boom]

I'm looking for any useful advice anyone may have.

[sailfish]

I don't have much to offer as you've already done a lot of diagnosing. I assume that you used the dye test in the coolant. I think that you've got it narrowed down, it's either a head or a head gasket. I would suggest that next you do a leak down test to determine which cylinder is the culprit. Then you'd have to remove the head and inspect for evidence of a leak.

Perhaps you know how to do a leak down but the test is accomplished by placing the piston at top-dead-center on the compression stroke and pressurizing the cylinder with an air compressor. In your case bottom-dead-center will work too, it's easier, the piston will naturally move there when you pressurize the cylinder. Just make sure that the valves are closed. Fill your radiator and watch it with the cap off. Give the test some time, like several minutes. I had a Subaru with a leaking head gasket and it took a few minutes for the cylinder to leak enough to start pushing out coolant. I didn't get bubbles but there was a big push of coolant. A big bubble would have appeared eventually. And of course I got results on the fourth cylinder (out of four) that I tested.

[N5XL]

Just a quick comment, the final torque is the value given in the manual PLUS 90 degrees. Are you sure the torque is correct?

[Boom]

@N5XL
Yeah, I'm aware of the plus 90 degrees

[sailfish]

More on the leak down test. It's usually done to look for loss of compression. Introduce air compressor pressure into the cylinder with an adapter hose and then start listening for where the air is going. The leaks will usually hiss. Leaking exhaust valves will hiss in the exhaust sustem, intakes in the carbs, and past the rings will hiss in the crankcase. Let's hope yours doesn't hiss in the radiator, that would be a big leak.