Yamaha XMAX YP125R - Service manual > Checking the valves and valve guides

The following procedure applies to all of the valves and valve guides.

1. Measure:

• Valve-stem-to-valve-guide clearance

Out of specification → Replace the valve guide.

Valve-stem-to-valve-guide clearance =

Valve guide inside diameter "a" -

Valve stem diameter "b"

Valve-stem-to-valve-guide clearance (intake) 0.010-0.037 mm (0.0004-0.0015 in)

Limit 0.080 mm (0.0031 in)

Valve-stem-to-valve-guide clearance (exhaust) 0.025-0.052 mm (0.0010-0.0020 in)

Limit 0.100 mm (0.0039 in)

2. Replace:

• Valve guide

TIP

To ease valve guide removal and installation, and to maintain the correct fit, heat the cylinder head to 100 ºC (212 ºF) in an oven.

a. Remove the valve guide with the valve guide remover "1".

b. Install the new valve guide with the valve guide installer "2" and valve guide remover "1".

Valve guide position

Intake 17.0-17.4 mm (0.670-0.685 in)

Exhaust 14.0-14.4 mm (0.551-0.567 in)

a. Valve guide position

c. After installing the valve guide, bore the valve guide with the valve guide reamer "3" to obtain the proper valve-stem-to-valve-guide clearance.

TIP

After replacing the valve guide, reface the valve seat.

Valve guide remover (ø4.5) 90890-04116

Valve guide remover (4.5 mm) YM-04116

Valve guide installer (ø4.5) 90890-04117

Valve guide installer (4.5 mm) YM-04117

Valve guide reamer (ø4.5) 90890-04118

Valve guide reamer (4.5 mm) YM-04118

3. Eliminate:

• Carbon deposits

(from the valve face and valve seat)

4. Check:

• Valve face

Pitting/wear → Grind the valve face.

• Valve stem end

Mushroom shape or diameter larger than the body of the valve stem → Replace the valve.

5. Measure:

• Valve margin thickness "a"

Out of specification → Replace the valve.

Valve margin thickness D (intake) 0.85-1.15 mm (0.0335-0.0453 in)

Valve margin thickness D (exhaust) 0.85-1.15 mm (0.0335-0.0453 in)

6. Measure:

• Valve stem runout

Out of specification → Replace the valve.

TIP

• When installing a new valve, always replace the valve guide.
• If the valve is removed or replaced, always replace the valve stem seal.

Valve stem runout 0.010 mm (0.0004 in)

Checking the valve seats

The following procedure applies to all of the valves and valve seats.

1. Eliminate:

• Carbon deposits (from the valve face and valve seat)

2. Check:

• Valve seat

Pitting/wear → Replace the cylinder head.

3. Measure:

• Valve seat width "a"

Out of specification → Replace the cylinder head.

Valve seat width C (intake) 0.90-1.10 mm (0.0354-0.0433 in)

Limit 1.6 mm (0.06 in)

Valve seat width C (exhaust) 0.90-1.10 mm (0.0354-0.0433 in)

Limit 1.6 mm (0.06 in)

a. Apply Mechanic's blueing dye (Dykem) "b" onto the valve face.

b. Install the valve into the cylinder head.

c. Press the valve through the valve guide and onto the valve seat to make a clear impression.

d. Measure the valve seat width.

TIP

Where the valve seat and valve face contacted one another, the blueing will have been removed.

4. Lap:

• Valve face
• Valve seat

TIP

After replacing the cylinder head or replacing the valve and valve guide, the valve seat and valve face should be lapped.

a. Apply a coarse lapping compound "a" to the valve face.

NOTICE

Do not let the lapping compound enter the gap between the valve stem and the valve guide.

b. Apply molybdenum disulfide oil onto the valve stem.

c. Install the valve into the cylinder head.

d. Turn the valve until the valve face and valve seat are evenly polished, and then clean off all of the lapping compound.

TIP

For the best lapping results, lightly tap the valve seat while rotating the valve back and forth between your hands.

e. Apply a fine lapping compound to the valve face and repeat the above steps.

f. After every lapping procedure, be sure to clean off all of the lapping compound from the valve face and valve seat.

g. Apply Mechanic's blueing dye (Dykem) "b" onto the valve face.

h. Install the valve into the cylinder head.

i. Press the valve through the valve guide and onto the valve seat to make a clear impression.

j. Measure the valve seat width again. If the valve seat width is out of specification, reface and lap the valve seat.

Checking the valve springs

The following procedure applies to all of the valve springs.

1. Measure:

• Valve spring free length "a"

Out of specification → Replace the valve spring.

Free length (intake) 41.71 mm (1.64 in)

Limit 39.62 mm (1.56 in)

Free length (exhaust) 41.71 mm (1.64 in)

Limit 39.62 mm (1.56 in)

2. Measure:

• Compressed valve spring force "a"

Out of specification → Replace the valve spring.

Installed compression spring force (intake) 140-162 N (14.28-16.52 kgf, 31.47-36.42 lbf)

Installed compression spring force (exhaust) 140-162 N (14.28-16.52 kgf, 31.47-36.42 lbf)

Installed length (intake) 35.30 mm (1.39 in)

Installed length (exhaust) 35.30 mm (1.39 in)

b. Installed length

3. Measure:

• Valve spring tilt "a"

Out of specification → Replace the valve spring.

Spring tilt (intake) 2.5º/1.8 mm

Spring tilt (exhaust) 2.5º/1.8 mm

See also: