diameter from the bore measurement. If the
precision measuring tools shown are not
available, the condition of the pistons and
bores can be assessed, though not quite as
accurately, by using feeler gauges as follows.
Select a feeler gauge of thickness equal to the
specified piston-to-bore clearance, and slip it
into the cylinder along with the matching
piston. The piston must be positioned exactly
as it normally would be. The feeler gauge
must be between the piston and cylinder on
one of the thrust faces (at right-angles to the
gudgeon pin bore). The piston should slip
through the cylinder (with the feeler gauge in
place) with moderate pressure; if it falls
through or slides through easily, the clearance
is excessive, and a new piston will be
required. If the piston binds at the lower end
of the cylinder, and is loose toward the top,
the cylinder is tapered. If tight spots are
encountered as the piston/feeler gauge is
rotated in the cylinder, the cylinder is
out-of-round (oval).
15 Repeat these procedures for the
remaining pistons and cylinder bores.
16 Compare the results with the
Specifications at the beginning of this
Chapter; if any measurement is beyond the
dimensions specified for that class (check the
piston crown marking to establish the class
of piston fitted), or if any bore measurement is
significantly different from the others
(indicating that the bore is tapered or oval),
the piston or bore is excessively-worn.
17 Worn pistons must be renewed; on some
engines, the pistons are available as Ford
replacement parts only as part of the
complete piston/connecting rod assembly.
See a Ford dealer or engine reconditioning
specialist for advice.
18 If any of the cylinder bores are badly
scuffed or scored, or if they are excessively-
worn, out-of-round or tapered, the usual
course of action would be to have the cylinder
block/crankcase rebored, and to fit new,
oversized, pistons on reassembly. See a Ford
dealer or engine reconditioning specialist for
advice.
19 If the bores are in reasonably good
condition and not excessively-worn, then it
may only be necessary to renew the piston
rings.
20 If this is the case, the bores should be
honed, to allow the new rings to bed in
correctly and provide the best possible seal.
Honing is an operation that will be carried out
for you by an engine reconditioning specialist.
21 After all the machining operations have
been carried out, the entire block/crankcase
must be washed very thoroughly with warm
soapy water to remove all traces of abrasive
grit produced during the machining
operations. When completely clean, rinse it
thoroughly and dry it, then lightly oil all
exposed machined surfaces to prevent
rusting.
22 The cylinder block/crankcase should now
be completely clean and dry, with all
components checked for wear or damage,
and repaired or overhauled as necessary.
Refit as many ancillary components as
possible, for safekeeping. If reassembly is not
to start immediately, cover the block with a
large plastic bag to keep it clean.
14 Main and big-end bearings -
inspection
4
1 Even though the main and big-end bearing
shells should be renewed during the engine
overhaul, the old shells should be retained for
close examination, as they may reveal
valuable information about the condition of
the engine (see illustration).
2 Bearing failure occurs because of lack of
lubrication, the presence of dirt or other
foreign particles, overloading the engine, and
corrosion. Regardless of the cause of bearing
failure, it must be corrected before the engine
is reassembled, to prevent it from happening
again.
3 When examining the bearing shells, remove
them from the cylinder block/crankcase and
main bearing caps, and from the connecting
rods and the big-end bearing caps, then lay
them out on a clean surface in the same
general position as their location in the
engine. This will enable you to match any
bearing problems with the corresponding
crankshaft journal. Do not touch any shell’s
bearing surface with your fingers while
checking it, or the delicate surface may be
scratched.
4 Dirt or other foreign matter gets into the
engine in a variety of ways. It may be left in
the engine during assembly, or it may pass
through filters or the crankcase ventilation
system. It may get into the oil, and from there
into the bearings. Metal chips from machining
operations and normal engine wear are often
present. Abrasives are sometimes left in
engine components after reconditioning,
especially when parts are not thoroughly
cleaned using the proper cleaning methods.
Whatever the source, these foreign objects
often end up embedded in the soft bearing
material, and are easily recognised. Large
particles will not embed in the material, and
will score or gouge the shell and journal. The
best prevention for this cause of bearing
failure is to clean all parts thoroughly, and to
keep everything spotlessly-clean during
engine assembly. Frequent and regular engine
oil and filter changes are also recommended.
5 Lack of lubrication (or lubrication
breakdown) has a number of inter-related
causes. Excessive heat (which thins the oil),
overloading (which squeezes the oil from
the bearing face) and oil leakage (from
excessive bearing clearances, worn oil pump
or high engine speeds) all contribute to
lubrication breakdown. Blocked oil passages,
which usually are the result of misaligned oil
holes in a bearing shell, will also starve a
bearing of oil, and destroy it. When lack of
lubrication is the cause of bearing failure, the
bearing material is wiped or extruded from the
shell’s steel backing. Temperatures may
increase to the point where the steel backing
turns blue from overheating.
6 Driving habits can have a definite effect on
bearing life. Full-throttle, low-speed operation
(labouring the engine) puts very high loads on
bearings, which tends to squeeze out the oil
film. These loads cause the shells to flex,
which produces fine cracks in the bearing
face (fatigue failure). Eventually, the bearing
material will loosen in pieces, and tear away
from the steel backing.
7 Short-distance driving leads to corrosion of
bearings, because insufficient engine heat is
produced to drive off condensed water and
corrosive gases. These products collect in the
engine oil, forming acid and sludge. As the oil
is carried to the engine bearings, the acid
attacks and corrodes the bearing material.
8 Incorrect shell refitting during engine
assembly will lead to bearing failure as well.
Tight-fitting shells leave insufficient bearing
running clearance, and will result in oil
starvation. Dirt or foreign particles trapped
behind a bearing shell result in high spots on
the bearing, which lead to failure.
9 Do not touch any shell’s bearing surface
with your fingers during reassembly; there is a
risk of scratching the delicate surface, or of
depositing particles of dirt on it.
15 Engine overhaul -
reassembly sequence
1 Before reassembly begins ensure that all
new parts have been obtained and that all
necessary tools are available. Read through
the entire procedure to familiarise yourself with
the work involved, and to ensure that all items
Engine removal and overhaul procedures 2D•21
14.1 Typical bearing failures
2D
1595Ford Fiesta Remake