Mazda MX-5 NA silhouetteMX-5 NA

INSPECTION / REPAIR

B-51prose procedure
Continued from B-50

Valve seat

  1. 1
    Inspect the contact surface of each valve seat and valve face for the following: (1) Roughness. (2) Damage.
  2. 2
    If necessary, resurface the valve seat with a 45° valve seat cutter and/or resurface the valve face.
    Resurfacing a valve seat with a 45° valve seat cutter inserted into the combustion chamber from above the cylinder head.
    05U0BX-130
  3. 3
    Apply a thin coat of Prussian blue to the valve face.
    Checking valve seat contact pattern by applying Prussian blue to the valve face and pressing it against the seat. Inset at right shows the resulting blue contact ring around the seat (good contact pattern).
    05U0BX-131
  4. 4
    Inspect the valve seating by pressing the valve against the seat. (1) If blue does not appear 360° around the valve face, replace the valve. (2) If blue does not appear 360° around the valve seat, resurface the seat.
  5. 5
    Measure the seat contact width.
    Single valve drawn upright with the seat contact band on its face labelled SEAT CONTACT WIDTH (bracket between the two contact lines on the valve face).
    05U0BX-132
  6. 6
    Verify that the valve seating position is at the center of the valve face. (1) If the seating position is too high, correct the valve seat with a 75° cutter and a 45° cutter. (2) If the seating position is too low, correct the valve seat with a 35° (IN) or 15° (EX) cutter and a 45° cutter.
    Valve seat angle geometry — two cross-sections labelled EX (exhaust) and IN (intake), each showing a 75°/45° (EX) and 75°/45° (IN) seat-cut profile above the 45° seating angle.
    05U0BX-133
  7. 7
    Seat the valve to the valve seat with lapping compound.
  8. 8
    Inspect the sinking of the valve seat.
    Measuring valve seat sinking with a depth gauge / dial indicator placed over the valve stem at the top of the cylinder head.
    05U0BX-134
Seat contact widthValve seat
0.8—1.4mm (0.031—0.055 in)
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