Sounds like a rod bearing to me. The noise you are hearing is the either the rod slapping the crank at TDC and BDC...or the piston smacking the cylinder head. Climb under the rig with engine idling..if the sound is amplified in the exhuast then its the second one. Time to start hunting for an engine or lining up your ducks for a rebuild. At the stage your engine is in (based on your description) simple bearing replacement is out of the question. Rods will need to be resized and the crank polished. If you wait to long and continue running the engine you can kiss any core value or rebuild options g-bye.

~Darin <img src="/forums/images/graemlins/baby.gif" alt="" />