Main Bearings - Derbi boulevard 125 4t Workshop Manual

Engine
Inspecting the crankshaft plain bearings
- To obtain a good bushing lubrication it is neces-
sary to have both an optimal lubricating pressure
(4 bar) and a good oil flow rate; the bushings must
be correctly positioned so as not to obstruct the oil
supply channels.
- The main bushings are comprised of two half-
bearings, one containing holes and channels for
lubrication and the other solid.
- The solid half-bearing is intended to stand the
thrusts caused by combustion, and for this reason
it is arranged opposed the cylinder.
- To prevent shutters in the oil feeding channels,
the matching surface of the two half-bearings must
be perfectly orthogonal to the cylinder axis, as
shown in the figure.
- The oil supply channel section is also affected by
the bushings driving depth compared with the
crankshaft axial clearance of the limiting surface.
N.B.
TO KEEP THIS POSITION OF THE BUSHINGS
ON THE CRANKCASE, FITTING IS FORCED ON
STEEL RINGS INSERTED IN THE CASTING OF
BOTH CRANKCASE HALVES.
Characteristic
Standard driving depth
1.35 ÷ 1.6
- Check the inside diameter of the main bushings in the three directions indicated in the diagram.
- Repeat the measurements for the other bushing half. see diagram.
- The standard bushing diameter after driving is variable on the basis of a coupling selection.
- The bushing housings in the crankcase are classified into 2 categories - Cat. 1 and Cat. 2 - just like
those for the crankshaft.
3 - The main bushings are subdivided into 3 thickness categories; see the table below:
N.B.
DO NOT TAKE THE MEASUREMENT ON THE TWO HALF-SHELL COUPLING SURFACE SINCE
THE ENDS ARE RELIEVED TO ALLOW BENDING DURING THE DRIVING OPERATION.
Specification
ENG - 128

MAIN BEARINGS

B
Desc./Quantity
Blue