Main Bearings; Inspection; Checking Clearances - MerCruiser GM4 Service Manual

Main Bearings

IMPORTANT: Before removing main bearing caps
or connecting rod caps, mark them for reassemb-
ly in their original locations.
Main bearings are of the precision insert type and do
not use shims for adjustment. If clearances are found
to be excessive, a new bearing, both upper and lower
halves, will be required. Service bearings are avail-
able in standard size and .001 in., .010 in. and .020
in. undersize.
Selective fitting of both rod and main bearing inserts
is necessary in production in order to obtain close tol-
erances. For this reason you may find one half of a
standard insert with one half of a .001 in. undersize
insert which will decrease the clearance .0005 in.
from using a full standard bearing.
When a production crankshaft cannot be precision
fitted by this method, it then is ground .009 in. under-
size on main journals only. A .009 in undersize bear-
ing and .010 in. undersize bearing may be used for
precision fitting in same manner as previously de-
scribed. Any engine, fitted with a .009 in. undersize
crankshaft, will be identified by the following mark-
ings:
".009" will be stamped on crankshaft counter-
weight forward of center main journal.
No. "9" will be stamped on block at left front oil pan
rail.
IMPORTANT: If crankshaft has an undersize jour-
nal and a new bearing is required, journal must be
reconditioned to accept a .010 or .020 in. under-
size bearing as .009 in. undersize bearings are not
available for service.

Inspection

In general, the lower half of the bearing (except No.
1 bearing) shows a greater wear and the most dis-
tress from fatigue. If, upon inspection, the lower half
is suitable for use, it can be assumed that the upper
half is also satisfactory. If the lower half shows evi-
dence of wear or damage, both upper and lower
halves should be replaced. Never replace one half
without replacing the other half.
Index
90-816462 2-695

Checking Clearances

To obtain accurate measurements while using Plasti-
gage, or its equivalent, engine must be out of the boat
and upside down so crankshaft will rest on the upper
bearings and total clearance can be measured be-
tween lower bearing and journal.
To assure the proper seating of the crankshaft, all
bearing cap bolts should be at their specified torque.
In addition, preparatory to checking fit of bearings, the
surface of the crankshaft journal and bearing should
be wiped clean of oil.
1. With the oil pan and oil pump removed, remove
bearing cap and wipe oil from journal and bearing
cap to be inspected.
2. Place a piece of gauging plastic the full width of
the bearing (parallel to the crankshaft) on the jour-
nal as shown.
b
a - Gauging Plastic
b - Journal
3. Install the bearing cap and evenly torque the re-
taining bolts to specifications. Bearing cap MUST
be torqued to specification in order to assure
proper reading. Variations in torque affect the
compression of the plastic gauge.
IMPORTANT: Do not rotate the crankshaft while
the gauging plastic is between the bearing and
journal.
4. Remove bearing cap. The flattened gauging plas-
tic will be found adhering to either the bearing cap
or journal.
a
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