Piston Slap and Cylinder Wall Clearance
Excessive piston to wall clearance causes a knocking sound known as piston slap. It's a problem with older high mileage engines. The clearance between the piston and the cylinder wall increases, causing the pistons to rock back and forth in their bores.
Most pistons are made of aluminum, and when they're cold, they are at their smallest. As the metal heats up, it expands and fits tighter in the cylinder bore. The clearance between the piston and the cylinder walls has decreased along with the noise.
Most pistons are oval-shaped, with more diameter around the thrust side than around the piston pin side. As the engine warms, it expands and fills in the piston pin side, making it round. These are called cam ground pistons.
Run a wet compression test to see if a cylinders seal is within specifications. If the compression gauge indicates an increase after adding oil, the piston rings are worn. If the readings change little or stay the same, the head gasket, a valve or its seat is faulty. Check for TSB's and any manufacturer's special procedures.