long story short, car overheats and the water pump goes. a loud sound like a "shotgun" goes off but yet the car doesn't die. i figure my rod went. so i start up the car after towing it home and it has a loud knocking sound and gets louder if you accelerate. check coolant overflow and no anitfreeze in there or the radiator. pour some water into the radiator and water is leaking throw the water pump. talk to my mechanic and he said to bring in the head and that the knocking sound is my broken rod. i take in the head and he says the valves are okay. i check myself and no valves are bent in any form. he checks the head surface and says it's warped to .001's.
i finally get the time to drop the motor and remove the oil pan and to my surprise there are no metal shavings at all. move the cranshaft via c.s bolt and rods are okay. no broken rods. so im now stuck at a lost. wtf happened and wtf is the knocking noise.. any feedback is greatly appreciated.
motor is a 90 b18a1 with only 137k...

 

Do all of the pistons move freely if u turn the crankshaft? Did u happen to look at any of the bearings or mains?

 

yes...all rods and pistons move freely.
looked at bearings and mains and they are fine. any way i can check to make sure there is no play in them?

 

The typical damage that occurs when a car overheats is usually either a warped/deck, or piston/ring damage. I'm guessing your combustion chamber got too hot and your pistons tried to expand more than the block would let them. I'm presuming you did some damage up in that area.

 

Well the bearings need to have a certain oil clearance spec..in order to check it u need to use some plastiguage and torque the bolts down to spec and then take off the caps again and measure the plastiguage and compare it with the blueprint specs...

But what sack master said is correct its gotta be something with the rings/pistons, take them out of the cylinder and check em out...