Valve Guides; Valve Seat Reconditioning - Mercury 25 EFI Service Manual

Valve Guides

Measure the valve guide inner diameter. Replace the cylinder head if the valve guide inner diameter is out of specification.
a
Valve Guide Inside Diameter
Intake
Exhaust

Valve Seat Reconditioning

1. Clean the carbon deposits from the combustion chambers and valve seats. Check the valve seats for pitting.
2. Several different types of equipment are available for reseating valve seats. Follow the equipment manufacturers
instructions.
3. Apply a thin even layer of mechanic's bluing dye (Dykem) onto the valve seat.
4. Insert the valve into the valve guide and lap the valve slowly on the valve seat.
5. Remove the valve and measure the valve seat contact pattern width. Resurface the valve seat if not in specification.
a
b
Valve Seat Contact Width
Intake And Exhaust Valve
6. To reface a valve seat, use a 30°, 45°, and 60° valve seat cutting tool.
a
b
NOTE: When twisting the cutter, keep an even downward pressure to prevent chatter marks.
NOTE: After refacing the valve seat or replacing the valve and valve guide, the valve seat and valve face should be
lapped.
7. Start with the 45° cutting tool to clean up any pitting or rough surface of the valve seat.
45°
8. Use the 30° cutting tool to adjust the contact width of the top edge of the valve seat.
a
90-8M0105568 eng MARCH 2016
c
c
a - Rough surface of valve seat
a
20954
a -
30°
20976
a - Valve guide inside diameter
20975
5.51–5.55 mm (0.217–0.218 in.)
5.51–5.57 mm (0.217–0.219 in.)
a -
b -
d
c -
20969
d -
a - 30° angle
b - 45° angle
c - 60° angle
20964
Previous contact width
© 2016 Mercury Marine
Correct valve seat contact width
Valve seat too high
Valve seat too low
Valve seat too wide
1.0–2.0 mm (0.0393–0.0787 in.)
Cylinder Head
Page 4B-15