Inspecting The Crankshaft Plain Bearings - VESPA GTS 250 I.E. Workshop Manual

Vespa GTS 250 I.E.
ERABLY DECREASE THE MAIN BUSHING
AND CONNECTING ROD LUBRICATION PRES-
SURE.
N.B.
THE HEAD LUBRICATION CHANNEL IS PRO-
VIDED WITH A SHUTTER JET; THIS GIVES A
"LOW PRESSURE" HEAD LUBRICATION; THIS
CHOICE WAS MADE TO REDUCE THE OIL
TEMPERATURE IN THE SUMP. THE JET
CLOGGING IMPAIRS THE HEAD LUBRICA-
TION AND THE TIMING MECHANISMS. A JET
FAILURE CAUSES A DECREASE OF THE MAIN
BUSHING AND CONNECTING ROD LUBRICA-
TION PRESSURE.

Inspecting the crankshaft plain bearings

- I
n order for the main bushings to be lubricated
properly it is necessary to have both an outstand-
ing lubricating pressure (3.2 bar) and a good oil
flow rate, therefore it is essential for the main bush-
ings to be positioned properly so that the oil feed
channels are not choked.
- The main bushings are comprised of two half-
bearings, one containing holes and channels for
lubrication and the other solid.
- The solid half bearing must withstand the thrusts
due to combustion and is therefore positioned op-
posite the cylinder.
- So as not to choke the oil feed channels it is es-
sential for the coupling surfaces of the two half
bearings to be perfectly at right-angles to the cyl-
inder axis as shown in the figure.
- The oil feed channels cross-section is also influ-
enced by the depth to which the bushings are
driven compared with the crankshaft axial clear-
ance containment surface.
- Check the inside diameter of the main bushings
in the three directions indicated in the diagram.
- Repeat the measurement for the other half of the
main bushing. see diagram.
Engine
ENG - 53