My 240sx has 191,000 miles on it, and up until a couple of weeks ago had only minor problems. I thought it had blown the head gasket (antifreeze in oil) but when I pulled the head the gasket didn't have any tracks. So I am assuming a cracked head or block. How do you figure out what and where might be cracked? I have done a good visual inspection of the head and don't see anything, and am wondering whether to just get a used engine. Also, I noticed the head gasket is missing a lot of cutouts for the cylinder cooling passages. The new gasket also does not include cutouts that would allow coolant flow. Why?

You'll have to find a place that can check to see if the head is cracked. they use a magnetic field and metal shavings to find cracks or something along those lines. I had a similar problem on a past car and I couldn't see the crack but the test they ran revealed a hair line crack which was invisible to the naked eye.

I would look around for a used engine with less miles. If it is the head and you get a new one something in the bottom end may be ready to go soon after you fix it and your car is down again.

hope i helped

http://www.gude.com/

Makes an aftermarket head for the 240.. I have no idea what kinda $$ tho..