1. Connecting Rod worn out. This happens from years of hard and long service. Replace the rod.
2. Piston is worn out. The skirt's clearance is too great and it flaps back and forth in the cylinder.
3. Excessive carbon deposits on the head. Remove head and clean. Clean carbon from piston head. PISTON must be at TDC to do this.
4. Engine is out of time. Take the engine to an authorized shop and have them time the engine.
5. Cylinder is worn out. When this happens, either oversize the cylinder to the next size or shortblock the engine.
6. Rod bolt loose. This can either be a factory defect or heat/lack of lubrication has made the bolts relax. Chances are the rod will need to be replaced and the crankshaft polished, or turned down.