ENGINE BEARINGS
If pressure lubrication was used and the pin to which the bearing
was to be fitted still had its original 2" diameter, one
would fit the bearing to the 2.002" section of the mandrel,
in which case they would be fitting the bearing with .002" clearance,
which is about correct for a pressure-lubricated bearing of this
size. Accordingly the smaller steps on the mandrel could be similarly
used to take care of the undersize and worn crankpins.
The ultimate success of any connecting-rod bearing job is largely
dependent on the amount of care used in reassembling the engine.
The most important item is, as before mentioned, alignment.
The mechanic should therefore use great care to see that each
of the connecting-rod assemblies is properly aligned on a suitable
alignment jig before attaching them to the crankshaft.
The connecting-rod aligning jig shown on page 803 is typical of
several that may be secured on the open market. In an emergency
where no jig is available, the mechanic can use the bearing-fitting
mandrel and a machinist's square for doing work.
If any rod shows more than .010" misalignment when tested
on the alignment, it should be straightened with a bending bar
or special straightening jig, which can usually be secured from
the maker of the alignment jig.
When attaching the rod assemblies to the crank-shaft, the mechanic
should make sure that they are being installed in the same relative
position as before which will be indicated by the marks made on
them previous to their removal.
In the case of new rods it will be necessary that they be marked
to correspond to the marks on the ones removed. Where the lower
rod bearing is off-set with respect to the longitudinal center
line of the rod the markings become doubly important.
After all the rod assemblies have been installed, it is advisable
to give a final check for alignment, as on page 1324 under "Checking
connecting-rod trueness," by observing top of rod while turning
engine slowly. If this test shows movement of top of rod across
bosses, it indicates a crankshaft crankpin that is not at right
angles to cylinder bore.
MAINTENANCE OF MAIN BEARINGS
The maintenance of crankshaft main bearings will be treated under
this heading.
If the special oil tank test or other testing method shows that
one or more main bearings are loose the mechanic should make a
further inspection before proceeding to the actual job of tightening.
This further inspection is suggested as a means to prevent damage
to the crankshaft caused from straining or springing as shown in
exaggerated form in Fig. 14, page 1323.
The first step in the inspection consists in removing the cap from
the loose main bearing, after which the babbitt face of cap bearing
is examined for scoring or burning, and the journal thoroughly
cleaned of oil and dried with a clean cloth.
Revolve shaft a few times to see that journal is dry, then apply
a fairly heavy coat of Prussian blue to the journal, as would be
clone for spotting a rod bearing. Now, with the cap still removed,
rotate the shaft two or three full turns, at the same time watching
the exposed journal.
If during the turning the coating of blue is evenly thinned (Fig.
23), it indicates that the shaft journal is making contact with
the upper hearing half and the adjustment can then be safely made,
either by removing shims or by lapping down the cap and lower shell,
as outlined for tightening rod hearings on a preceding page under
the heading "Adjustment of connecting-rod bearings for clearance," page
1325.
Fig.23. Degree of contact between main bearing journal (M) and
ton bearing (B) is determined without removing crankshaft, as shown.
If none of the Prussian blue coating has been re-moved or thinned,
it indicates that the upper bearing babbitt surface is not contacting
with the journal and the shaft should be removed for renewal of
the faulty upper half bearing. Exceptions to this are on those
cars where interchangeable type main bearings are used, in which
rase the upper half bearing can often be renewed by dropping the
shaft only far enough to remove old bearing and slip in a new one.
On some engines equipped with interchangeable type main bearings
the manufacturers recommend that the engine be removed from the
frame and the crankshaft taken out in orderto renew the main bearing
upper half bearing. The new bearing half, either upper or lower,
is then slipped into place without scraping or reaming and the
shaft reassembled to engine.
Other manufacturers, using interchangeable type main bearings,
such as Oakland, Oldsmobile, Chrysler, Overland
ii, and wills Sainte Claire, state that upper half bearings can he renewerl simply by
loosening all caps, dropping shaft just enough to slide out upper
half bearings and inserting new ones. The engine in this case is
not removed from the frame, nor is the shaft removed from the engine.
In either case it is advisable to recheck the condition of the
newly installed upper half bearings with Prussian blue, so as to
detect a possible under size journal or a bearing with too much
babbitt which would prevent the other journals from contacting
with their upper half bearings.
During this test it is necessary that the main bearings on each
side of the one being tested be fairly tight, so as to hold the
shaft up against the top half bearings. If the blue is removed
or thinned on one side of the journal only, it indicates an out-ofround
condition which can be corrected only by removing shaft and retruing
that journal.
Checking the Crankshaft
Assuming that the crankshaft is removed, because of a misaligned
upper main bearing half or one or more badly scored crankpins,
next test will be to determine condition of all the main bearing
journals.
In checking the condition of the shaft journals the mechanic will
make use of the micrometer or special outside dial gauge, applying
the micrometer at not less than three points along the length and
not less than six points on the diameter of each journal. The method
of procedure is shown in Fig. 16, pg. 1324.
If any of the journals or pins are found to be scored or more than
.003 out of round, the shaft should be reconditioned by grinding
or turning be-fore attempting to fit new bearings.
Assuming now that the crankpins and journals are in good shape,
the next test before proceeding to the actual job of refitting
bearings is to check the shaft for alignment. The alignment test
should al-ways be made after the circularity test, as will be seen
in the next paragraphs.
The alignment test should be made preferably from the end journals
rather than the center holes in the ends of the shaft, as the latter
method cannot always be depended on for accurate results. The method
of making the test is illustrated at Fig. 24.
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