Inspect the block for cracks, corrosion, or damage to the threads in the holes. A special workshop can check for hidden cracks. If defects are found, the block must be repaired, or if this is not possible, replaced.
Check the cylinder bores for wear or scratches.
Measure the diameter of each cylinder hole just below the roller, at the center and at the bottom.
Next, measure the diameter of each cylinder at the same points, but along the crankshaft axis. Compare with the technical data.
If there are no precise measuring instruments, the clearance between the piston and the cylinder wall can be measured with a feeler gauge.
The feeler gauge must be inserted between the wall and one of the piston support surfaces (at an angle of 90° to the piston ring hole).
The piston must slide along the cylinder (with the dipstick inserted) under moderate pressure.
If the piston falls through or slides very easily when the feeler gauge is at the maximum clearance value, the clearance is greater than the allowable value and a new piston is required. If the piston is stuck at the bottom of the cylinder but moves freely at the top, the cylinder is taper. If the feeler gauge gets stuck in some places when moving around the circumference, the cylinder is out of round.
Repeat this operation for the remaining pistons and cylinders.
If the cylinder walls are heavily worn or scratched, the taper or out-of-roundness exceeds the permissible value (0.01 mm), the block must be bored and honed in a workshop. After boring, pistons and rings of a repair (larger) size must be used.
If the cylinders are in satisfactory condition, then it is enough to hone them.
(The original material is located on the website «BMWMAN»)