Mercury 305 CID, Service Manual, Page: 227 / 967

ENGINES - 3A-35 90-823225--1 1096
2. Inspect cylinder head gasket surface for burrs,
nicks, or erosion or other damage. Also, check
flatness of cylinder head gasket surface, using a
machinist’s straight edge and feeler gauges as
shown. Refer to “Specifications.”
72307
a - Straight Edge
b - Feeler Gauge
c - Take Measurements Diagonally Across Head Both Ways)
and Straight Down Center Head
b
a
IMPORTANT: Cylinder head-to-block gasket sur-
face should be resurfaced if warped more than
specified. When head resurfacing is required,
cylinder head-to-intake manifold gasket surface
on head must be milled to provide proper align-
ment between intake manifold and head.
3. Inspect valves for burned heads, cracked faces
or damaged stems.
IMPORTANT: Excessive valve stem to bore clear-
ance will cause excessive oil consumption and
possible valve breakage. Insufficient clearance
will result in noisy and sticky valves.
4. Measure valve stem clearance as follows:
a. Attach a dial indicator to cylinder head, posi-
tioning it against the valve stem and close to
the valve guide.
b. Holding valve head off seat about 1/16 in. (2
mm), move valve stem back and forth in di-
rection shown. Compare stem clearance with
specifications.
c. If clearance exceeds specifications, it will be
necessary to ream valve guides for oversized
valves, as outlined under “Valve Guide Bore
Repair.”
72336
a - Valve Stem
b - Dial Indicator
c - Valve Guide
b
c
a
Valve Guide Bore Repair
IMPORTANT: Be sure to measure valve stem di-
ameter of both the intake and exhaust valve, as
valve stem diameter may or may not be the same
for both valves.
If .015 in. oversize valves are required, ream valve
guide bores for oversize valves, as follows:
1. Measure valve stem diameter of old valve being
replaced and select proper size valve guide
reamer from chart below.
Standard Valve
Stem Diameter
Reamer Required for
.015 In. Oversize Valve
.341 In. J-5830-2
.372 In. J-7049