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Subaru Impreza 3 / Impreza WRX / Impreza WRX STI. Manual - part 178

ME(w/o STI)-93

Cylinder Block

MECHANICAL

2. 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 temper-

ature 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)

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 tempera-

ture of 20°C (68°F).
Cylindricality:

Limit

0.015 mm (0.0006 in)

Out-of-roundness:

Limit

0.010 mm (0.0004 in)

(A) Main journal size mark
(B) Cylinder block (RH) – (LH) combination mark

(E) #3 cylinder bore size mark

(F) #4 cylinder bore size mark

ME-00170

#5
#4
#3
#2
#1

(A)

(B)

(F)
(D)

5
4

(C)

(E)

(A) Piston pin direction
(B) Thrust direction

H1: 10 mm (0.39 in)
H2: 45 mm (1.77 in)
H3: 80 mm (3.15 in)
H4: 115 mm (4.53 in)

ME-04734

(A)

(B)

ME(w/o STI)-94

Cylinder Block

MECHANICAL

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 tempera-

ture 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)

5) Calculate the clearance between cylinder and

piston.
NOTE:

Measurement should be performed at a tempera-

ture 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 clear-

ance 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 cylin-

der 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 100.005 mm (3.9372 in)

3. PISTON AND PISTON PIN

1) Check the piston and piston pin for damage,

cracks or wear. Replace if faulty.

2) Check the piston ring groove for wear or dam-

age. Replace if faulty.

3) Make sure that the piston pin can be inserted

into the piston pin hole with a thumb at 20°C (68°F).

Replace if faulty.
Clearance between piston pin hole and piston

pin:

Standard

0.004 — 0.008 mm (0.0002 — 0.0003 in)

ME-00172

ME-00173

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ME(w/o STI)-95

Cylinder Block

MECHANICAL

4) Check the snap ring installation groove (A) on

the piston for burr. If necessary, remove burr from

the groove so that the piston pin can lightly move.

5) Check the snap ring for distortion, cracks and

wear.

4. PISTON RING

1) If the piston ring is broken, damaged or worn, or

if its tension is insufficient, or when the piston is re-

placed, replace the piston ring with a new part of

the same size as piston.
NOTE:

• The top ring and second ring have the mark to

determine the direction for installing. When install-

ing the ring to piston, face marks to the top side.

• Oil ring consists of the upper rail, expander and

lower rail. When installing oil ring on piston, be

careful of the direction of each rail.

2) Using the piston, insert the piston ring and oil

ring into the cylinder block so that they are perpen-

dicular to the cylinder wall, and measure the piston

ring gap using a thickness gauge.

3) Fit the piston ring straight into the piston ring

groove, then measure the clearance between pis-

ton ring and piston ring groove with a thickness

gauge.
NOTE:

Before measuring the clearance, clean the piston

ring groove and piston ring.

(A) Upper rail
(B) Expander

ME-00175

(A)

ME-02480

(A)

(B)

(C)

(A)

(B)

(C)

Standard

mm (in)

Piston ring gap

Top ring

0.20 — 0.25

(0.0079 — 0.0098)

Second ring

0.37 — 0.52

(0.015 — 0.020)

Oil ring rail

0.20 — 0.50

(0.0079 — 0.0197)

Standard

mm (in)

Clearance between
piston ring and pis-
ton ring groove

Top ring

0.040 — 0.080

(0.0016 — 0.0031)

Second ring

0.030 — 0.070

(0.0012 — 0.0028)

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ME(w/o STI)-96

Cylinder Block

MECHANICAL

5. CONNECTING ROD

1) Replace the connecting rod, if the large or small

end thrust surface is damaged.

2) Check for bend or twist using a connecting rod

aligner. Replace the connecting rod if the bend or

twist exceeds the limit.
Limit of bend or twist per 100 mm (3.94 in) in

length:

0.10 mm (0.0039 in)

3) Install the connecting rod with bearings attached

to the crankshaft, and using a thickness gauge,

measure the thrust clearance. If the thrust clear-

ance exceeds the standard or uneven wear is

found, replace the connecting rod.
Connecting rod thrust clearance:

Standard

0.070 — 0.330 mm (0.0028 — 0.0130 in)

4) Inspect the connecting rod bearing for scar,

peeling, seizure, melting, wear, etc.

5) Measure the oil clearance on each connecting

rod bearing using plastigauge. If any oil clearance

is not within the standard, replace the defective

bearing with a new part of standard size or under-

size as necessary.
Connecting rod oil clearance:

Standard

0.017 — 0.045 mm (0.0007 — 0.0018 in)

6) Inspect the bushing at connecting rod small end,

and replace the connecting rod if it is worn or dam-

aged.

7) Measure the piston pin clearance at connecting

rod small end. If the measured value is not within

the standard, replace the connecting rod and piston

pin as a set.
Clearance between piston pin and bushing:

Standard

0 — 0.022 mm (0 — 0.0009 in)

(A) Thickness gauge
(B) Connecting rod

(A)

(B)

ME-05475

ME-00180

Unit: mm (in)

Bearing

Bearing size

(Thickness at center)

Outer diameter of

crank pin

Standard

1.490 — 1.506

(0.0587 — 0.0593)

51.976 — 52.000

(2.0463 — 2.0472)

0.03 (0.0012)

Undersize

1.504 — 1.512

(0.0592 — 0.0595)

51.954 — 51.970

(2.0454 — 2.0461)

0.05 (0.0020)

Undersize

1.514 — 1.522

(0.0596 — 0.0599)

51.934 — 51.950

(2.0447 — 2.0453)

0.25 (0.0098)

Undersize

1.614 — 1.622

(0.0635 — 0.0639)

51.734 — 51.750

(2.0368 — 2.0374)

ME-00181

ME-00174

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