Cylinder And Piston

WARNING: This page is about a different variant/trim than selected.

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: 92.005 - 92.015 mm (3.6222 - 3.6226 in)

B: 91.995 - 92.005 mm (3.6218 - 3.6222 in)

Fig 1: Identifying Engine Stamp Information
Courtesy of SUBARU OF AMERICA, INC.

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).

Calculate the cylindricality of cylinder liner by using the following formula.

Calculation formula

C = The larger value between the calculation values C' and C"
C' = (D(a)-D(b))/2
C" = (D(c)-D(d))/2

C: Cylindricality of cylinder liner
D(a): The largest value of all the values obtained by measuring the cylinder liner inner diameter in the direction of the piston pin
D(b): The smallest value of all the values obtained by measuring the cylinder liner inner diameter in the direction of the piston pin
D(c): The largest value of all the values obtained by measuring the cylinder liner inner diameter in the thrust direction
D(d): The smallest value of all the values obtained by measuring the cylinder liner inner diameter in the thrust direction

Calculate the out-of-roundness of cylinder liner at each measurement height by using the following formula.

Calculation formula

R = (D(c)-D(d))/2

R: Out-of-roundness of cylinder liner
D(c): The larger value between the measurement values in the piston pin direction and in the thrust direction of cylinder liner inner diameter
D(d): The smaller value between the measurement values in the piston pin direction and in the thrust direction of cylinder liner inner diameter

Cylindricality:

Limit

0.030 mm (0.0012 in)

Out-of-roundness:

Limit

0.030 mm (0.0012 in)

Fig 2: Checking Out-Of-Roundness Of Cylinder Liner

Courtesy of SUBARU OF AMERICA, INC.

When the piston is to be replaced due to general or cylinder wear, select a suitable sized piston by measuring the piston clearance.
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:

37.3 mm (1.4685 in)

Piston outer diameter:

Standard

A: 92.005 - 92.015 mm (3.6222 - 3.6226 in)

B: 91.995 - 92.005 mm (3.6218 - 3.6222 in)

0.25 mm (0.0098 in) oversize

92.245 - 92.265 mm (3.6317 - 3.6325 in)

0.50 mm (0.0197 in) oversize

92.495 - 92.515 mm (3.6415 - 3.6423 in)

Fig 3: Measuring Outer Diameter Of Piston
Courtesy of SUBARU OF AMERICA, INC.
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 (-0.0004 - 0.0004 in)
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.
NOTE: Immediately after reboring, the cylinder diameter may differ from its real diameter due to temperature rise.

Thus, when measuring the cylinder diameter, wait until it has cooled to room temperature.

Cylinder inner diameter boring limit (diameter):

To 92.515 mm (3.6717 in)