8 VN 53 | JANUARY – MARCH 2019
KEEPING IT
CLEAR
Glyco elaborates on the importance of bearing
clearances in rebuilt engines.
Rebuilding engines should always take place in a sanitary
environment to avoid “building-in” dirt. With processes like engine
block re-bore and crankshaft re-grind, the job itself produces dirt
and debris which will cause problems when carried around by the
oil. Hence, all components (especially engine block oil galleries, oil
ways and crankshaft) need to be clean before assembly.
OILED UP
Bearing surfaces and journals must be
thoroughly lubricated during assembly. Oil
should run all over bearings, blocks and caps
until every surface is fully covered. It should
never be spread with a finger or a rag. Once
complete, check the lubrication system is
working and replacement components match
the manufacturer’s specification.
A bearing with insufficient lubrication
oil to the bearing surfaces. If the journal
diameter has worn to below the minimum size
stated in the Glyco catalogue, the crankshaft
must be reground to the next undersize.
Cleanliness is paramount. Dirt trapped behind
the steel back of the bearing shell will reduce
clearance. Cap bolts should be tightened in
accordance with recommended torque figures.
If manufacturer’s specifications are
unavailable, a general guide is:
A bearing eroded by dirt
Friction causes the shaft to ride up the bearing
as the shaft’s rotation drags oil into the
wedge formed between the two surfaces. Oil
is forced towards the thin edge of this wedge
and the pressure pushes the shaft away from
the bearing surface. As the speed of the shaft
increases, the oil pressure rises until it is high
enough to support the load on the shaft, when
it begins to force the shaft towards the centre.
Crankshaft Journal Clearances
• Minimum oil clearance = 0.0005" (1/2
thou") per inch of journal diameter
• Maximum oil clearance = the minimum oil
clearance calculated as above, plus 0.002"
Crankshaft End Float Clearances
CRANKSHAFT JOURNAL SIZE
END FLOAT
2" – 2.75" diameter 0.004" – 0.006"
0.006" – 0.008"
0.008" – 0.010"
BEARING CLEAR DO IT RIGHT 2.75" – 3.5" diameter
Sufficient bearing clearance means good
conformability between the bearing and the
crankshaft journal. Too little clearance will
cause the oil to be squeezed out between
crankshaft and bearing, leading to friction
and wear. The oil pressure in the shaft can be up to
several thousand pounds per square inch
(psi), compared to a pressure of just 60psi
from the oil pump. Therefore, bearing
clearance must be to the manufacturer’s
specification or the pump can’t circulate the 3.5" and over
For engines with speeds in excess of 6,000
revs/min, always refer to the manufacturer for
details of crankshaft journal and crankshaft
end float oil clearances.
The Glyco range comprises over 1,500 OE
quality parts available via the F:Drive or your
local FPS representative for same/ next-day
delivery. For more information on the best
way to avoid issues when replacing bearings
visit https://fpsdistribution.com/
replacing-bearings/.
Friction between the bearing and crankshaft