Cylinder And Piston

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

The cylinder bore size is stamped on the cylinder block's front upper surface.
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: Measuring Diameter
Courtesy of SUBARU OF AMERICA, INC.
How to measure the 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 Fig 2 , using a cylinder bore gauge.

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

Taper:

Standard

0.015 mm (0.0006 in)

Out-of-roundness:

Standard

0.010 mm (0.0004 in)

Fig 2: Checking Out-Of-Roundness
Courtesy of SUBARU OF AMERICA, INC.
When the piston is to be replaced due to general or cylinder wear, determine a suitable sized piston by measuring the piston clearance.
How to measure the outer diameter of each piston:
Measure the outer diameter of each piston at the height as shown in Fig 3 . (Thrust direction)

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

Piston grade point H:

37.0 mm (1.457 in)

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 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).

Cylinder to piston clearance at 20°C (68°F):

Standard

-0.010 - 0.010 mm (-0.00039 - 0.00039 in)
Boring and honing

If the value of taper, out-of-roundness, or cylinder-to-piston clearance measured exceeds the standard value or if there is any damage on the cylinder wall, rebore it to use an oversize piston.
CAUTION: When any of the cylinders needs reboring, all other cylinders must be bored at the same time, and use oversize pistons. Do not perform boring on one cylinder only. Nor use an oversize piston for one cylinder only.
If the cylinder inner diameter exceeds 100.015 mm (3.9376 in) 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, pay attention when measuring the cylinder diameter.