Piston Selection

Important:  The coating on the piston allows for an interference fit between the cylinder and the bore. The piston diameter can NOT be measured accurately because the piston coating is not a consistent thickness. Do NOT measure the piston diameter.

To select the correct piston for installation, the cylinder bore must be measured. If the cylinder bore diameter is within service specifications, install the original piston/connecting rod assembly or a new, standard size piston/connecting rod assembly. A used piston/connecting rod assembly may be reinstalled if, after cleaning and inspection, the piston is not damaged. If the cylinder bore is NOT within specifications, the cylinder must be resized to accept a new, oversized piston.

For proper piston fit, the engine block cylinder bores should not have excessive wear or taper.

1. Inspect the engine block cylinder bore. Refer to Engine Block Cleaning and Inspection .
2. Inspect the piston/connecting rod assembly for damage. Refer to Piston, Connecting Rod, and Bearings Cleaning and Inspection .
3. Use the J 8087 and measure the cylinder bore diameter. Refer to Engine Block Cleaning and Inspection .
   Fig 1: Measuring Cylinder Bore Diameter Using J 8087

   Courtesy of GENERAL MOTORS CORP.
4. Measure the J 8087  with a micrometer and record the reading.
5. Compare the cylinder bore measurement to the specifications. Refer to Engine Mechanical Specifications .
   1. 5.1. If the cylinder bore is within specifications, select the original piston or a new, original size piston.
   2. 5.2. If the cylinder bore is not within specifications, select the next oversized piston/connecting rod assembly, then bore and hone the cylinder bore to fit the oversize piston.
   Fig 2: Measuring J 8087

   Courtesy of GENERAL MOTORS CORP.