Yamaha XMAX YP125R - Service manual > Cylinder and piston (YP250R)

Removing the cylinder and piston

Removing the piston

1. Remove:

• Piston pin clips "1"
• Piston pin "2"
• Piston "3"

NOTICE

Do not use a hammer to drive the piston pin out.

TIP

• Before removing the piston pin clips, cover the crankcase opening with a clean rag to prevent them from falling into the crankcase.
• Before removing the piston pin, deburr the piston pin clip groove and the piston pin bore area.

  If both areas are deburred and the piston pin is still difficult to remove, remove it with the piston pin puller set "4".

Piston pin puller set 90890-01304

Piston pin puller YU-01304

2. Remove:

• Top ring
• 2nd ring
• Oil ring

TIP

When removing a piston ring, open the end gap with your fingers and lift the other side of the ring over the piston crown.

Checking the cylinder and piston

1. Check:

• Piston wall
• Cylinder wall

Vertical scratches → Rebore or replace the cylinder, and replace the piston and piston rings as a set.

2. Measure:

• Piston-to-cylinder clearance

a. Measure cylinder bore "C" with the cylinder bore gauge.

TIP

Measure cylinder bore "C" by taking side-to-side and front-to-back measurements of the cylinder.

Then, find the average of the measurements.

Bore 69.000-69.005 mm (2.7165- 2.7167 in)

Taper limit 0.030 mm (0.0012 in)

Out of round limit 0.030 mm (0.0012 in)

"C" = maximum of D1-D2

"T" = maximum of D1 or D2 - maximum of D5 or D6

"R" = maximum of D1, D3 or D5 - minimum of D2, D4 or D6

b. If out of specification, rebore or replace the cylinder, and replace the piston and piston rings as a set.

c. Measure piston skirt diameter D "a" with the micrometer.

b. 5 mm (0.20 in) from the bottom edge of the piston

Piston

Diameter D

68.965-68.980 mm (2.7152- 2.7157 in)

d. If out of specification, replace the piston and piston rings as a set.

e. Calculate the piston-to-cylinder clearance with the following formula.

Piston-to-cylinder clearance =

Cylinder bore "C" -

Piston skirt diameter "D"

Piston-to-cylinder clearance 0.020-0.040 mm (0.0008-0.0016 in)

Limit 0.15 mm (0.0059 in)

f. If out of specification, rebore or replace the cylinder, and replace the piston and piston rings as a set.

Checking the piston rings

1. Measure:

• Piston ring side clearance

Out of specification → Replace the piston and piston rings as a set.

TIP

Before measuring the piston ring side clearance, eliminate any carbon deposits from the piston ring grooves and piston rings.

Piston ring

Top ring

Ring side clearance 0.040-0.080 mm (0.0016- 0.0031 in)

Limit 0.130 mm (0.0051 in)

2nd ring

Ring side clearance 0.030-0.070 mm (0.0012- 0.0028 in)

Limit 0.130 mm (0.0051 in)

2. Install:

• Piston ring (into the cylinder)

TIP

Level the piston ring into the cylinder with the piston crown.

a. 5 mm (0.20 in)

3. Measure:

• Piston ring end gap

Out of specification → Replace the piston ring.

TIP

The oil ring expander spacer end gap cannot be measured. If the oil ring rail gap is excessive, replace all three piston rings.

Piston ring

Top ring

End gap (installed) 0.15-0.30 mm (0.0059-0.0118 in)

Limit 0.55 mm (0.0217 in)

2nd ring

End gap (installed) 0.30-0.45 mm (0.0118-0.0177 in)

Limit 0.80 mm (0.0315 in)

Oil ring

End gap (installed) 0.20-0.70 mm (0.0079-0.0276 in)

Checking the piston pin

1. Check:

• Piston pin

Blue discoloration/grooves → Replace the piston pin, and then check the lubrication system.

2. Measure:

• Piston pin outside diameter "a"

Out of specification → Replace the piston pin.

Piston pin outside diameter 16.991-17.000 mm (0.6689- 0.6693 in)

Limit 16.971 mm (0.6681 in)

3. Measure:

• Piston pin bore diameter "b"

Out of specification → Replace the piston.

Piston pin bore inside diameter 17.004-17.015 mm (0.6694- 0.6699 in)

Limit 17.045 mm (0.6711 in)

4. Calculate:

• Piston-pin-to-piston-pin-bore clearance

Out of specification → Replace the piston pin and piston as a set.

Piston-pin-to-piston-pin-bore clearance =

Piston pin bore diameter "b" -

Piston pin outside diameter "a"

Piston-pin-to-piston-pin-bore clearance 0.004-0.024 mm (0.0002-0.0009 in)

Checking the timing chain guide

1. Check:

• Timing chain guide (exhaust side) Damage/wear → Replace.

Installing the piston and cylinder

1. Install:

• Top ring "1"
• 2nd ring "2"
• Upper oil ring rail "3"
• Oil ring expander "4"
• Lower oil ring rail "5"

TIP

Be sure to install the piston rings so that the manufacturer marks or numbers face up.

2. Install:

• Piston "1"
• Piston pin "2"
• Piston pin clips "3"

TIP

• Apply engine oil to the piston pin.
• Make sure the arrow mark "a" on the piston points towards the exhaust side of the cylinder.
• Before installing the piston pin clips, cover the crankcase opening with a clean rag to prevent the clips from falling into the crankcase.

3. Lubricate:

• Piston
• Piston rings
• Cylinder (with the recommended lubricant)

Recommended lubricant Engine oil

4. Offset:

• Piston ring end gaps

a. Top ring

b. Upper oil ring rail

c. Oil ring expander

d. Lower oil ring rail

e. 2nd ring

f. 20 mm (0.79 in)

A. Intake side

5. Install:

• Dowel pins
• Cylinder gasket
• Cylinder

TIP

• While compressing the piston rings with one hand, install the cylinder with the other hand.
• Pass the timing chain and timing chain guide (intake side) through the timing chain cavity.

See also: