Brand new rebuilt TPI 305. Machine shop assembled the short block and did the rest. I thought I put it all together correctly. Was VERY careful. Fired it up and it was running quite good. Then it gets worse and slows idle to a sputter and I hear a small knock in the bottom. Shut it down immediately. thought maybe bad oil pump? Peeked thru the oil filler cap and I see a green slime looking substance sitting on top of the head in the valley next to a valve. Looks like coolant and oil mix. The oil on the dipstick looks OK still but I probably didn't run it enough for a lot of coolant to get in the crank case yet. Pretty sure that is coolant sitting on top of that head.

How does this happen? The obvious thought is a F'd up intake gasket but I am soooooo sure I did it right. I suppose its the old Sherlock Holmes train of thought? If everything else is eliminated but one thing, not matter how unlikely the one thing is it must be the answer? Any thoughts?

 

Coolant into the oil sounds like head gasket. Drain the oil and find out if coolant is in it. Oil floats on water so the dipstick may not show it if the two were still separated enough. I'd be surprised if that much coolant got in there that quickly to destroy bearings. I would think something else went wrong if it's knocking. Now way to know with out tests or tear down.

 

My thoughts are 2 options. #1, need silicone on the "China wall" (the center of the intake manifold area on the block) or my other thought is the block is cracked in the water jackets. I had a 350 motor I bought from a wrecked iroc. I broke it down and had it hot tanked and that's when the machine shop found the crack, block was trash. The old man at the machine shop said it's common for these motors cuz they cheaped out on materials when casting them so all the water jackets near the ports for the rods are thin and once the motor gets too hot they crack. My father (who was an ase gm mechanic when these cars rolled out onto the lot) said the guy was right

 

Did you use thread sealant on the head bolt threads? If not that is the issue. The head bolts go into the water jacket.

RBob.