Cylinder And Piston

1. The cylinder bore size is stamped on the front upper face of the cylinder block.
   NOTE:

   • Measurement should be performed at a temperature of 20°C (68°F).
   • Standard sized pistons are classified into two grades, "A" and "B". These grades should be used as guide lines in selecting a standard piston.

   Standard diameter: 

   A: 99.505 - 99.515 mm (3.9175 - 3.9179 in) 

   B: 99.495 - 99.505 mm (3.9171 - 3.9175 in) 

   Fig 1: Identifying Standard Size Pistons

   Courtesy of SUBARU OF AMERICA, INC.
2. Measure inner diameter of each cylinder.

   Measure the inner diameter of each cylinder in both the thrust and piston pin directions at the heights as shown in the figure, using a cylinder bore gauge.

   NOTE: Measurement should be performed at a temperature of 20°C (68°F).

   Cylindricality: 

   Standard 

   0.015 mm (0.0006 in) 

   Out-of-roundness: 

   Standard 

   0.010 mm (0.0004 in) 

   Fig 2: Measuring Inner Diameter Of Each Cylinder

   Courtesy of SUBARU OF AMERICA, INC.
3. When the piston is to be replaced due to general or cylinder wear, select a suitable sized piston by measuring the piston clearance.
4. Measure outer diameter of each piston.

   Measure the outer diameter of each piston at the height as shown in the figure. (Thrust direction)

   NOTE: Measurement should be performed at a temperature of 20°C (68°F).

   Piston grade point H: 

   38.2 mm (1.50 in) 

   Piston outer diameter: 

   Standard 

   A: 99.505 - 99.515 mm (3.9175 - 3.9179 in) 

   B: 99.495 - 99.505 mm (3.9171 - 3.9175 in) 

   0.25 mm (0.0098 in) oversize 

   99.745 - 99.765 mm (3.9270 - 3.9278 in) 

   0.50 mm (0.0197 in) oversize 

   99.995 - 100.015 mm (3.9368 - 3.9376 in) 

   Fig 3: Measuring Outer Diameter Of Each Piston

   Courtesy of SUBARU OF AMERICA, INC.
5. Calculate the clearance between cylinder and piston.
   NOTE: Measurement should be performed at a temperature of 20°C (68°F).

   Clearance between cylinder and piston at 20°C (68°F): 

   Standard 

   -0.010 - 0.010 mm (-0.00039 - 0.00039 in) 

6. Boring and honing

1. If any of the measured value of cylindricality, out-of-roundness or cylinder-to-piston clearance is out of standard or if there is any damage on the cylinder wall, rebore it to replace with an oversize piston.
   CAUTION: When any of the cylinders needs reboring, all other cylinders must be bored at the same time, and replaced with oversize pistons.
2. If the cylinder inner diameter exceeds the limit after boring and honing, replace the cylinder block.

Cylinder inner diameter boring limit (diameter): To 100.005 mm (3.9372 in)