Main Bearings; Checking Clearances - MerCruiser 305 CID (5.0L) Service Manual

5.0l / 305 cid / 5.7l / 350 cid ENGINES

Main Bearings

IMPORTANT: Before removing main bearing caps or connecting rod caps, mark them
for reassembly 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 available 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 to obtain
close tolerances. 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 is then ground
.009 in. undersize ON ONLY THOSE MAIN JOURNALS THAT CANNOT BE PROPERLY
FITTED. ALL JOURNALS WILL NOT NECESSARILY BE GROUND UNDERSIZE. A .009
in. undersize bearing and .010 in. undersize bearing may be used for precision fitting in the
same manner as previously described.
Inspection
In general, the lower half of the bearing (except No. 1 bearing) shows a greater wear and
the most distress 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 evidence of wear
or damage, both upper and lower halves should be replaced. Never replace one half without
replacing the other half.

Checking Clearances

To obtain accurate measurements while using Plastigage, or its equivalent, engine must be
out of the boat and upside down so crankshaft will rest on the upper bearings and total clear-
ance can be measured between lower bearing and journal.
To assure the proper seating of the crankshaft, all bearing cap bolts should be at their speci-
fied 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 journal as shown.
Do not rotate the crankshaft while the gauging plastic is between the bearing and
journal.
Index
Page 3A-60
a - Gauging Plastic
b - Journal
NOTICE
a
b
90-861327--1 OCTOBER 1999
SERVICE MANUAL NUMBER 24
72357